This happened a few times in my game. I remember seeing something similar reported.
This was NOT just after someone quit.
The only thing I could think is players not accepting a trade that was offered. I took one screenshot when the turn seemed fairly long at 30 seconds for a bot.
I took another screenshot 19 seconds later. Perhaps after 10 seconds a bot should resolve the trade rather than running down to 5 seconds on the timer.
In this case the turn took all the way down to 5 seconds, which is too long a turn considering they only offered one trade and built nothing that round.