The problem of homeless people setting fires that bring down bridges and create other issues has been growing for some time. And it is likely to get worse as the homeless problem grows as far back as the early 1990s, the Oakland Hill firestorm was traced to two homeless men who were sleeping up in the hills, and they were trying to keep warm and make coffee. Even in a heatwave, the California Coast gets rather cold at night. The poor guys had even tried to do the right thing. They knew there was fire danger, but their options were risk-freezing or making a fire. So after they made their coffee and things warmed up, they carefully put the fire out and buried it how you are supposed to. Under most conditions, that would have been enough. Still, it got so hot that day (I know I was working overtime on Saturday and thinking, "This is really dangerous fire weather.") It smoldered undetected for about 24 hours and then burst into flames on Sunday morning. I do wonder if something like that is happening with the bridges. And not all homeless people try to be as careful (or even know how to be as careful as those guys were).
It would also be a great "cover" for intentional terrorism, even done once removed. Pay some homeless guys a pittance (or give them drugs) in exchange for "forgetting" to put out their fire correctly or do it in a certain place. But my guess is, unless evidence comes up to say otherwise, these are accidents.
Accidents will get increasingly worse as the homeless population grows. And it will be compounded by people stealing copper, cables, and anything else they think they can sell or use to support themselves. Not all homeless people are drugged-burnout cases that laying around waiting to die. Some are, but some are not.