The answer to this question is complicated!
I already had a go-round on this in PoliGAF a little while back, so I'll try to make it quicker this time.
We had a huge speculative bubble in real estate in the 2000s. Prices and values just kept climbing and climbing. So a lot of people made a lot of very aggressive investments in real estate, because, hey, if it keeps climbing it's free money! Part of this speculative bubble included mortgages, which after all are a real estate investment. So a lot of aggressive mortgages got given out. This might seem unwise, but if you know the value of the house is just going to keep going up forever, it's actually pretty safe.
Of course, like all speculative bubbles, eventually the bubble popped and housing prices collapsed, which caused a bunch of these investments to fail, which caused a bunch of institutions to end up losing money, which caused other institutions who loaned them money to end up at risk of losing more money, etc., etc., and suddenly you have an interdependent group of institutions all at risk of collapsing as a group and blowing up the financial system.
Once we got through dealing with all of that, a bunch of judges went back and looked at those aggressive mortgages and determined that, hey, without the context of a speculative bubble those were pretty dumb mortgages to write, and so charged a bunch of banks with fraud as a result. Since, again, at the time the mortgages were written, people assumed the asset values would just keep going up and so the mortgages would be fine, this might be a little harsh. Fraud requires foreknowledge. The behavior of the banks doesn't really suggest that they knew the mortgages would fail -- they mostly held onto them and lost money on them. But, you know, a lot of people did get fucked during the financial crisis and it might be nice to try to raise some money to help them, which we can easily do by fining banks billions of dollars, right? After all, they're the ones who have the money.
So that's fine as far as it goes. Personally, while I don't really have a problem with charging a bunch of banks with crimes and making them pay a bunch of fines which we can use to help people whose mortgages went underwater, because fuck banks basically, I think that it is actually just incorrect, in a way that will make it harder for you to understand financial regulation issues, to think that the CAUSE of the financial crisis was fraud. The fundamental cause of the crisis was the speculative bubble! That's why Dodd-Frank makes it harder for banks to get overleveraged to try to reduces the consequences of the bubble in the future.