Thanks to Doc's serial-killer-like penchant for decorating his room with old newspaper clippings, the audience is informed that his house burned down -- but it's never explained how that happened. All we know is that after the fire Doc sold his property to developers, presumably working for Burger Duke.
Ah, yes, remember when you went through that devastating tragedy and then
framed the newspaper articles about it?
Here's where things start to get suspicious: The news article also describes Doc as a "bankrupt inventor," which is weird, because earlier in the movie he told Marty that he'd spent his entire family fortune building the time machine. The paper is dated 1962 -- so Doc must have used up all of his money in just seven years, leaving him bankrupt and desperate. Since Shark Tank didn't exist in 1955, torching the Brown mansion may have been the only way Doc could think to raise the funds for his time machine.
In doing so, Doc would get the insurance money as well as the money from the sale of the property. And he needed both, because even after getting all of that money, by 1985 his bachelor pad/filthy garage is full of "past due" bills. It doesn't help that a big part of his budget goes to buying dozens of clocks for no apparent reason.
Most of those bills are from the clock store.