Possible bug: units spontaneously fortify

[jpm7734]

Hi, I am running BTS 3.19 and I am experiencing an odd issue. Each turn after the build prompts cycle through and the first active unit is selected, all units fortify themselves, workers, land military, navy, etc., without my having given them any commands. I can go to each unit or stack manually, select it, then issue commands as normal, but unless I do this the game will simply sit and wait for me to end my turn. I am completely at a loss. I have been running 3.19 successfully since immediately after it came out. The only thing that is different is that I tried to play an online game for the first time right before this began, though I doubt that has anything to do with it. Does anyone know what's going on?