I would guess one of the most common cause is things electrical. If I owned an indoor dry stack I would make it a requirement that each boat have a batt disconnect and launch crew would turn batts off before putting a boat in the rack.
Bilge pumps running dry, stereos and stereo amps, blower motors, light fixtures... All these I have seen smoking in boats, but no fire.. yet.
Also a few starters engaging due to moisure in ign switches. Those will crank engines until something burns up.
Once a fire starts in these things, it will be dang hard to stop as y'all have noted in posts above.
Best to not have a fire start at all.
Batts off, risk much lower.