Somewhere shortly after 02:38 my minotaur shot a projectile behind the direction its turret was facing.

The engine has a system in place to control burst fire patters as one shot. What happens with minataur is that because it's fire pattern is a burst of 2, and it uses the in-built engine features to accomplish that, it does indeed only check for the "ok to fire" for the first round of the burst. On every game frame when your unit has a target within it's firing range, the engine calls functions in the unit script to confirm that the enemy is still within firing range, and to prepare for firing (often rotating the turret, sometimes opening the weapon). Once the check for "preparing to fire weapon" returns that the weapon is ready, the engine starts calling after-firing functions and spawning projectiles, like the animation for cannon recoil. Importantly, most burst weapons stop processing the checks for turret rotation or projectile friendly fire collisons, which have caused problem in the past with weapons like cerberus (if you're an old and attentive player, you've probably seen this). You can take advantage of this with units like Merlin which have long firecycles, or most famously dante D-gun.

At least the bug where cannons shoot 90 degrees orthogonal to the cannon got squashed many years ago. Funny Bertha murderholes.

I mean whatever the solution is, this is currently broken. Just a matter of time until someone has a widget to make minotaurs always shoot their second shoot at enemies no matter how they are driving.