It seems a lot of the time agnostics misuse the term "agnostic" and use it to mean "non-aggressive atheist". Most agnostics will say they don't believe in god, Which makes them an atheist, Not agnostic.
If we look at what the word agnostic means we see it's definition is; A person who believes the existence of god is unknowable. Now, This isn't to say that they "don't know" if god exists, It is to say that regardless if god exists or not, They are of the belief that it impossible to know.
So given that the existence of god is unknowable either way, What possible rationale could they have to believe god doesn't exist? For Atheists the rationale is simple, There is no evidence. But for someone who believes that even if god does exist it would be impossible to know a lack of evidence can not be a contributing factor to their disbelief, Because their belief is that even if there is a god there is no way to know.
I personally believe that if god did exist we would know about it, Which makes a lack of evidence a reasonable reason for my disbelief. But if you don't believe a god exists and call yourself agnostic you need as much faith as someone who claims god does exist.