akenaten's suggestion does have the merit that if it's possible that it could have happened that way, it doesn't sound implausible, unlike the other two which I mentioned. The question is whether an accidental hit on a store of chemical weapons in the course of a bombardment could have these consequences. In principle it should be possible for the inspectors to determine that, and whether it appears to have happened.

Such a store could either be one set up by rebels, or possibly a previously existing government storage facility.

And there is a real danger that even if such an accidental explosion was the cause, there are people, in Syria and elsewhere, who positively want to see outside military engagement escalated, as a way to achieve political objectives.