Atheism isn't a response to anything. You don't need to be aware of God to lack a belief in Him. If no-one ever mentioned the concept of God then I expect everyone would be an atheist.
Do you believe in the eight-eyed hexapoids of Merak four? Probably not -- (I just made them up).
Is your a-hexapoidism a response to the concept of hexapoids, or did you lack a belief before I mentioned them?
I'll bet you've always been an a-hexapoidist.
Believing in something you have no evidence of is irrational.
Not believing in something you have no evidence of is rational.
Theists believe in something they have no evidence of.
Atheists lack belief in something they have no evidence of.
So which is the rational position?
I think the problem with is that two different ideas are conflated in a single word: atheism.
atheism can refer either to a lack of belief
OR a disbelief
This is a subtle but significant distinction.
The absence of an idea leaves nothing to accept or reject (believe or disbelieve), but the presence of an idea gives something to accept or reject (believe or disbelieve).
So if you lack belief in a god or gods (also known as weak atheism), then you aren't explicitly asserting that there are none.
Therefore, weak atheism is not a response to theism.
But if you have a disbelief in a god or gods (also known as strong atheism), then you have taken the extra step to explicitly deny.
Therefore, strong atheism is a response to theism.
In the case of the hexapoids, I might respond that I had never thought about hexapoids before and therefore had no occasion to believe or disbelieve in their existence and that now that you have mentioned them I can say that I don't have a belief that they exist (which doesn't mean that I believe that they do not exist). I might argue that just because you made them up, doesn't mean they don't actually exist somewhere.
But I could also respond, instead, that I think the notion of hexapoids is absurd and that I don't simply lack a belief in their existence, but that
I actually believe that they do not exist. I might claim that your statement that you "just made them up" is sufficient proof that they cannot possibly exist.
So since atheism is a word that has multiple meanings, it is pointless to discuss whether or not atheism is a response to theism without first indicating which meaning you are using.
So what about the problem put forth in the OP?
He thought atheism was rational but fells that a lot of atheists are irrational.
Atheists are like everyone else, I am not sure why you would expect any different.
The question then becomes: In what way are they "like everyone else"?