Dropping the phone didn't cause it to explode (if, in fact, it actually did explode). Continuing to use it after it was damaged may have caused an explosion. That's on the user, not the manufacturer. There are circuits built into the phone to protect against voltage surges, when the phone is physically damaged it's always going to be possible those circuits are damaged, there's not really any good way to prevent that, other than for the user to stop using the phone until they get it checked out.
The one thing I can think of to do would be for the phone to detect dropping and impact (which they can do with the accelerometers/gryroscopes, and completely disable the phone to make it inoperable. Shut it down. Of course, the damage may prevent that process from working correctly or completing, and it wouldn't work if the phone is turned off when the damage happens. And there are other ways to damage a phone that don't involve dropping/impact damage, for example dousing in a bucket of water (my wife did this one...ugh), shaking/vibrating, smoke damage, or using it in an extremely hot/cold/humid environment.
You can't prevent accidents, but as a consumer you have to be smart about using damaged goods where there is power or battery involved. That applies to mobile phones just as it does toaster ovens, cameras, or whatever.
