What you are describing in the OP tends to be the behavior of relatively recent atheists.
When one breaks out of indoctrination, dogmatic thinkings, comes to the conclusion that the evidence for the existence of gods is vastly insufficient, the philosophical arguments are all flawed, etc, etc, their newly found freedom of thought tends to point out how gullible their previous worldview was.
It's kind of hard not to want to point out just how unjustified theists beliefs are.
This can make them come off like dicks, or they might even be dicks.
Most recent atheists settle down after some time, and become more understated.