This is something you can accidentally do due to a command conflict with area commands, and how area commands are issued.
Some area commands target ground, where as other area commands target units. The primary area target command that targets units is Guard, using circle guard features and variants. The way an area command targets a unit is by doing a ground target, detecting if a unit is at the ground coordinates of the center of this area command, then issuing the command in relation to the detected unit. This is why it can be difficult to do an area guard on a moving unit, because if you take too long to issue the order, the unit will have moved away from the center of your command.
Reclaim can be used on many different classifications of targets. metal reclaim, energy reclaim, live units and units in construction. Area modifiers are used to help you target only a certain classification of target. E+Ctrl targets only metal reclaim, and E+Ctrl on a unit only targets live units. This can cause you to accidentally order your units to reclaim your live units in the area when an area command gets targeted to a specific unit. Video aid for your understanding.
https://streamable.com/wjka7s
