This may be a small bug caused by an endless cycle when game calculating some actions of creatures during their turn. Or animations. Or interactions. Or, as an option, something is wrong with your team. Like, unacceptable position of soldiers. Or countless lines of sight. Glitches with some devices or skills, such as ‘night vision’ or ‘motion detector’. Something unacceptable between yours and new aliens positions. The game, after finishing the movement of aliens, begins to transfer data to your soldiers to transmit the turn - and booom.
So you can simplify equipment to find the most stable team. Or try to rearrange your soldiers different ways before passing the turn. Sometimes it helps. However, most often you have to save yourself by the’win’ command.
@N7-Bandit it’s been mentioned in a couple of other threads that the likely cause is the Synedrion motion detector unit. Certainly when I unequipped all mine the problem stopped entirely.
I don’t know how it works with the other helmet units, I haven’t dared try…
Nobody said me this, when I was newbie 
I stumbled across this by accident. Glad I did, it was getting rather frustrating 
I found this as one of possible reasons too late, and now I just don’t care
yes it seems like the motion detector is doing it! i tried putting back on the second set i looted; witch is the problem! it desapears when i put it on, after i switch soldiers! excuse my bad english its not my language.
@N7-Bandit your English is perfectly understandable! I answer lots of emails every day from Americans and Australians - believe me your English is much better than thiers 
Happy gaming! 
Sorry, what was it now? Is this some kind of accusation or demand? Some extremely destructive speech about yourself / nation stereotypes, I just can’t see any reasons for it.
Yep, it partially clarifies - on some things you really should not pay attention, just step back and turn away for not to provoke progression of it by feeding it with your attention.