[1] http://www.nfpa.org/news-and-research/fire-statistics-and-re...
Batteries can explode when internally shorted, I had quite a few of 18650's explode on me over the years (usually through a fault of my own) the explosion isn't like a party popper it can be quite big and violent, and while explosions are some what not uncommon the battery flaming out completely when internally or externally shorted is. The Tesla has a few 1000's of 18650's cells even if a small number of those short during an accident it can cause allot of damage.
Another important difference between batteries and gas is that emergency workers (and anyone who's involved in the accident) can identify when the gas is leaking out, it's also much easier to stop the gas from catching on fire as wetting the vehicle or using other fire retardants will usually prevent the gas from catching fire. On the other hand you can have an internally shorted battery pack which can catch fire or explode at any moment and you would have no clue that it can happen.
Now don't take me wrong this isn't some anti-Tesla or anti-EV scare BS, this isn't some conspiracy either but saying that there are no valid concerns about the safety of the battery pack is ludicrous. We know how petrol cars work, we've been using them for 100+ years, we know when to look for a gas leak, we know how to deal with an oil/engine fire, EV's are a whole new ball game, eventually there will be a case where a Tesla or any other EV gets into a car accident and there will be a hot terminal somewhere in that car due to a short and someone will going to get zapped, at some point one of those batteries will explode and cause casualties, and many other things will happen that will be unique to EV's.
Using water on a gasoline or oil fire is a bad idea (the fire spreads since gas and oil are lighter than water), so fighting vehicle fires for ICE vehicles where the fuel tank has ruptured is a non-trivial problem for firefighters. The "easy" vehicle fires are ones where just the engine compartment and maybe the interior are on fire, but the fuel lines and tank are still in tact. In some ways a battery fire is easier to put out since it won't spread the same way and responds well to water.
Exploding 18650s tends to only happen with cheap, poorly manufactured variants. Most of the time if they short they will overheat, bulge, and sometimes catch on fire. Having enough of them actually explode to cause injury to vehicle occupants is probably about as likely as a fuel tank exploding in an ICE vehicle accident. Granted, there might be a higher fire risk for electric vehicles in severe collisions that damage the undercarriage area where the batteries are stored.