The jump from Vanilla to BTS

[wodan11]

reason 3 is improvements in the AI may now recognize the situation and do things such as run all artist specialists, which in most cases would generate sufficient culture to prevent the flip.

[Father Beast]

OK, it must be the units. I didn't know a bunch of units would prevent it. That explains why the AI is keeping about 10 units in this city which has starved down to size 1. because the cities in question weren't mine before, and the culture is abominably low and continues to drop.

[Boracks]

Yup, if a city is threatened by culture and has a chance to go into revolt, you can add units until the % chance drops to nothing.

To see the chance, in the city screen, hover the mouse pointer over the population percentages in the lower left (where it will say something like 3% Greek, 96% German). If there's a chance to revolt, it will appear with a percentage chance each turn.