It's all incremental. Here's some ancient history; some dates might be wrong.
1997: a commercial game, Total Annihilation, is released (by Cavedog Entertainment, led by Chris Taylor). It introduces a lot of concepts such as physically simulated weapons and streaming economy. One of the reasons for its longevity is that it is highly moddable.
1998: UberHack is created by BraveSirRobin. It is a simple mod for TA with some design changes (like anti-air only targetting planes) and fixes, and becomes a popular base for other modders' work. A lot of standalone units start being created and distributed over the TA community, usually in various unit packs.
1999: TA Demo Recorder is created by a clan called Swedish Yankspankers. It is basically a mod that allows to record games and play them back (something the retail game lacked).
2001: Swedish Yankspankers figure out that TA has orthogonal camera and no ability to rotate it, which makes TA's 3D world look 2D. They decide to evolve the demo recorder into a 3D engine able to use TA content.
2001: Absolute Annihilation (AA) is created by Caydr. It is a decently popular TA unit pack mod based on UberHack.
2004: first public release of Swedish Yankspankers' engine, by this time called TASpring (which eventually became just Spring). There's a separate infrastructure from TA (including a lobby server that makes it easy to install mods). AA is ported to Spring and becomes the #1 most popular mod there. People learn about Spring from TA communities.
2006(ish): Balanced Annihilation (BA) is forked from AA (by Noize), focusing on competitive 1v1 play. Also quickly becomes the #1 mod because the author of AA left.
2007:
Saktoth is amazed after some noob pwns him in 1v1 BA with 1 Dart by driving around his factory. Decides to make a mod where physical unit interactions are important, Complete Annihilation (CA), and gathers a bunch of enthusiastic players to help (these are just Spring players he met in the lobby, mostly). 
[LCC]quantum[0K] helps with game design (they come up with the idea of unit roles and balancing guidelines to keep units unique); 
Licho creates autohosts that ease keeping the mod played and 
detrino designs an update system which automatically downloads game updates, which allows very quick development. 
[LCC]jK creates the GUI framework and 
CarRepairer creates GUI. 
Histidine (aka KingRaptor) contributes in all areas, especially screwing up balance (or so I heard).
2010:
GoogleFrog proposes a redesign of CA (merging CA's two factions into one alongside the needed factory changes). This becomes ZK. Licho creates further infrastructure (website, later on the server) which lets ZK become a standalone game.
1997: a commercial game, Total Annihilation, is released (by Cavedog Entertainment, led by Chris Taylor). It introduces a lot of concepts such as physically simulated weapons and streaming economy. One of the reasons for its longevity is that it is highly moddable.
1998: UberHack is created by BraveSirRobin. It is a simple mod for TA with some design changes (like anti-air only targetting planes) and fixes, and becomes a popular base for other modders' work. A lot of standalone units start being created and distributed over the TA community, usually in various unit packs.
1999: TA Demo Recorder is created by a clan called Swedish Yankspankers. It is basically a mod that allows to record games and play them back (something the retail game lacked).
2001: Swedish Yankspankers figure out that TA has orthogonal camera and no ability to rotate it, which makes TA's 3D world look 2D. They decide to evolve the demo recorder into a 3D engine able to use TA content.
2001: Absolute Annihilation (AA) is created by Caydr. It is a decently popular TA unit pack mod based on UberHack.
2004: first public release of Swedish Yankspankers' engine, by this time called TASpring (which eventually became just Spring). There's a separate infrastructure from TA (including a lobby server that makes it easy to install mods). AA is ported to Spring and becomes the #1 most popular mod there. People learn about Spring from TA communities.
2006(ish): Balanced Annihilation (BA) is forked from AA (by Noize), focusing on competitive 1v1 play. Also quickly becomes the #1 mod because the author of AA left.
2007:
Saktoth is amazed after some noob pwns him in 1v1 BA with 1 Dart by driving around his factory. Decides to make a mod where physical unit interactions are important, Complete Annihilation (CA), and gathers a bunch of enthusiastic players to help (these are just Spring players he met in the lobby, mostly). [LCC]quantum[0K] helps with game design (they come up with the idea of unit roles and balancing guidelines to keep units unique); Licho creates autohosts that ease keeping the mod played and detrino designs an update system which automatically downloads game updates, which allows very quick development. [LCC]jK creates the GUI framework and CarRepairer creates GUI. Histidine (aka KingRaptor) contributes in all areas, especially screwing up balance (or so I heard).2010:
GoogleFrog proposes a redesign of CA (merging CA's two factions into one alongside the needed factory changes). This becomes ZK. Licho creates further infrastructure (website, later on the server) which lets ZK become a standalone game.