Having established as fact Clinton signed the law and Frank and Waters enabled it's abuse, here's why imo
Frank saw poor people getting to live in homes they could not afford. He knew they couldn't afford them, he knew there would be repossessions. The left views government money as endless, they believe the U.S. is exempt from fiscal laws that bankrupted many world leading countries before.
I believe Frank knew they would be foreclosed at some point, but they would have gotten to live for years at government expense and when they sold the houses would have risen in price and it was possible they would still walk away with an equity check + the free living time. I don't believe Frank anticipated the snowball effect take down of the world economy for which he will positively have a large share of the blame when unbiased historians look into the facts without the current political spin or emotion.
One thing is fact, the majority of people are uneducated and wrong on the topic if they blame Bush for the consequences of a law Clinton signed and the left allowed unsupervised exploitation of in the market place. Blame a group of greedy bankers, which is true on the surface but those bankers were given the vehicle and free rein by the left.
EXTRA CREDIT: How many of those evil greedy type bankers has Obama put in jail (Corzine is an example of the genre).
Answer: None, in fact with low interest to bank loans he continues to funnel trillions of middle class money to the banks in exchange for their tacit support, that is the fascist model of government.