Well that critical mixture is more of a range. It doesn't have to be a perfect mixture, just close. There has to be enough combustible density for ignition energy to pass from one particle to another, and enough oxygen density so that the combustibles can cause combustion. This is close to how a fuel air bomb works. However it's likely that the explosion heard wasn't directly related to the hydrogen at all. (skip to the 4th paragraph)
Note that the combustible density has to be high for stable/weak combustibles, or it can be lower for unstable/strong combustibles. Hydrogen is rather unstable (doesn't need much activation energy) and thus the particles can be really close, or relatively spread out.
Not all of the combustible has to go at the same instant, an explosion in one part might cause the density in another part to change, or could collectively give enough energy to cause ignition. Some things not normally considered explosive can become explosive if given the proper conditions (including a pressure vessel). Sugar dust, wheat dust, flour, and sawdust have been known to cause explosions in very rare conditions.
A pressure vessel failing is essentially an explosion, so it could be that the tank failed, and it just happened to be filled with this flammable gas, which then caused a fire. An explosion in the tank is nearly impossible, simply because there is no oxygen. Most combustibles need some sort of a pressure vessel to cause a proper explosion, if the ignition velocity is not fast enough, then it will cause a big fireball (think Hollywood explosion) but won't cause much explosion damage.
The real safety problem with using compressed hydrogen as a vehicle fuel is that it is a compressed gas. Even if a tank failure doesn't cause a lot of damage, the hydrogen will come out very quickly, leading to a possible fire. This isn't a problem with gasoline as it is a liquid, and on top of that only the vapors are really flammable. I don't know the safety ratings on a hydrogen tank put in a car, but the tanks would have to be done very well to be equal in safety to a gasoline tank (which can have inner bladders, that don't work with compressed hydrogen).
Hydrogen stored in metal hydrates is very stable, and easily safer than gasoline. But I don't think the energy density is high enough yet for practical use.
This post ended up being a lot longer and more of a ramble than I intended, sorry.