I noticed that while I was playing, when a mob was set on fire all of them ran around randomly, however, pigs did not and remained stationary.

After testing with a handful of passive mobs (eg. cow, sheep, rabbit, panda, wolf, chicken), all of them ran when set on fire by flint and steel, however, pigs moved very little or not at all, while I expected them to run as well.

This happened every time I used flint and steel on the block underneath a pig.