It's mind control, it's been buggy all along, being relatively uncommon
But yeah, mind controlled soldiers do, temporarily, act as alien units, which means they also provide sight and act like spotters. Somehow the information provided by the first mind-controlled guy made the Praetor decide to try and control another, which fails. I didn't dig deep into the AI logic, and it might be amplified by the fact that there are multiple psionic aliens on the map, but that's what happens anyway.
I'll build a new exe this evening - have to customize it first to disable the 0.35 features that you cannot run. With that exe, you should no longer encounter this problem, so if it freezes again, then there's yet another bug that your game makes show up.