What's causing the decline in religion and the increase in atheism?

The rise in atheism is due to parents have relinquished their responsibilities to teach their kids about God and Faith, and they have allowed our Public School teachers to alter history and ignore the role that religion has played since the Pilgrims left Europe. And it hasn't helped that the Supreme Court misinterpreted the First Amendment and banned prayer in our schools. 

It's not so much an increase in atheism as a lack of affiliation with any particular church.  People still believe in God.

The increase in people's critical thinking skills.

False dichotomy, also sadly not true.  About forty years ago it really looked like the whole world was becoming more secular.  Nowadays, fundamentalism is an important force in politics in all sorts of countries, including the US, Australia, Russia, Poland, India and Uganda.

Trump.
People are looking at him and thinking "If that is what Christians are like, I'd rather not be one"

Don't be deceived by mere appearances! It is that the form of religion is morphing. Surely you know that the worship of sticks and stones has now become the worship of stocks and shares? And atheists are right up there, adoring the god of Mammon, sacrificing their lives to obtaining more money that they then spend in Mammon's temples of shopping malls and car sales-rooms. Whatever is the priority in your life that you are devoted to, is your god. Atheists have no idea what worship is, you see, supposing it must involve rituals in a church and tithing to a religion etc, but they are full of their own beliefs about what to do by way of devotion to their monetary god. Or, for others, it's the goddess Sex. Lots of old gods on the block, just in more subtle disguises.

It's a cycle.
Peeps were super religious twenty years ago and now the youth are rebelling against what the generation before them did. 
That will all chance in 20 more years

common sense......its quite simple

Ted, I agree, and please add, although you implied this in your answer, that to learn not to believe superstitious remarks that offer no actual evidence, and be honest.

Education and intellectual sophistication.  Also, many religions are infested with clergy that steal money, engage in illicit sex, and lie; so people don't follow the preachers as much as they once did.