Atheists are angry at God because they think He should have done something for them in their pasts.
The idea that atheists are angry in general is itself already quite popular, but the myth that atheists are angry specifically at God for not having done what they want may be more popular. This combines the idea that atheists are angry, that they are prideful enough to think that they know better than God, and that they are rebelling against what God wants for the world. Atheists are thus attacked from a variety of convenient directions, except for the fact that the claim is all wrong.
The most obvious reason why everyone should recognize that this myth is false is the simple fact that you can't be angry at God unless you believe that God exists. This means that anyone who is angry at God must be a theist, not an atheist an atheist is a person who doesn't believe in any gods at all. Atheists therefore cannot be angry at any gods.
The only way an atheist could experience anger in the context of a god would be like how someone might be angry at a character in a novel but in such a case a person isn't really angry at the character, but experiencing anger in a more diffuse and undirected manner. You can't really be angry at a fictional character any more than you can love or hate a fictional character. The experience of such emotions in the context of fiction certainly doesn't cause a mentally balanced adult to start believing that the characters actually exist.
This means that it's logically impossible for atheists to be angry at god for any reason, but it's not the only basis for refuting this myth. We can also point to the empirical fact that atheists don't report, discuss, or experience this alleged anger. Unless religious theists who believe this myth also believe that atheists are engaged in a massive, coordinated conspiracy to cover up the truth, we must assume that if atheists as a whole were angry at God then there would be significant and clear evidence for this fact. We'd see atheists discussing it amongst themselves, for example, in the innumerable forums and groups where atheists spend their time.
Not only does no such evidence exist, but when asked directly atheists deny that they experience such anger at God. At most, some atheists may report that they first started down the road to atheism when they experienced some contradiction between what the expected from God and what actually happened. It might have been a trivial matter, like getting something for Christmas, or it might be something major like people suffering around the world. It might have induced anger in this person or it might have simply induced disappointment.
Whatever the case may be, it's unusual for atheists to keep experiencing these emotions or to still regard the experience as a foundational to their current atheism. It may have inspired them to start questioning their religious beliefs and being exercising greater skepticism towards everything they had previously taken for granted, but by now their atheism is based on more substantive arguments and ideas than whatever might have gotten the ball rolling years ago.
If this myth that atheists are angry at God after not getting something they wanted is both logically impossible and not at all supported by any available evidence, why on earth do people not only believe it but regularly assert with such conviction? They can't arrive at this as a logically necessary conclusion from the nature of atheism and they certainly didn't conduct any surveys that demonstrated common attitudes among atheists, either in their community or in the world generally.
The only source of such a myth must be prejudice: prejudiced assumptions about the character and rationality of atheists generally lead people to believe the worst things about atheists. Some religious theists hold atheists in such low regard that they are willing to believe anything about them, but that still doesn't quite explain how this particular myth would take hold. It may be that some people are projecting: perhaps they have experienced anger at God in the past and considered atheism, but never made that step and now assume that this must be the case for all atheists.