We do not yet have proof of the origin or intent of the gas attacks
The possibility of these being false flag operations has already been mentioned and absent any further evidence I think the probably quite high that this is the case.
The attacks were very small in the context of chemical warfare. They were ineffective from any military perspective: they repelled no advance, they denied no ground, they did not degrade any effective force through personnel or command and control structure, they achieved no military objective whatsoever.
The possible reasons they were small and ineffective would include:
1) The previously mentioned tip toeing on the "red line". This would appear to me to be an exercise in stupidity if true, so I would rate it as a low probability.
2) The use of old, defective, or damaged rounds, or the deployment of poorly trained or inexperienced personnel. It would seem that any even marginally competent military unit could identify these munitions defects quite readily. The idea of willfully dispensing such munitions to untrained units is absurd on its face for many technical reasons. It is however possible (and more likely in my opinion) that these weapons fell into the hands of civilians (rebels?) by theft or fortuitous capture of a stockpile who were not trained in their use. (Since these are currently speculated to be gas filled mortar rounds, no heavy or sophisticated systems are needed to deploy them.) Several rebel factions have serious differences where the loss of some of the opposing faction might be viewed as a collateral benefit while attempting to bring new players into the conflict. The only real reason to fire a couple of rounds and scoot (which seems to be the attackers M.O.) is because sticking around long enough to be identified and positioned would result in your destruction. Rebels sneaking into the margins of army territory to give evidence of firing location would be such a group. Friendly forces on the army side would have far less to fear.
3) With no determinable military objective, it seems rational and prudent to consider other possible objectives which might even include agents of a foreign power who seek to advance their own agenda or solidify a position.
Hasty action on our part with large gaps in our intel at this point could lead to very undesirable consequences. Action should be taken but it would serve us well to be certain it is the correct and appropriate action. Let's never see an historical footnote that says "2013 -the year that [such and such a group/nation] played the United States like a fiddle.