Holder and the Obama DOJ were likely involved in illegal activities running guns to Mexico and Latin America in what appears to be an effort to push up statistics on firearms traceable to US sources in an effort to push the treaty through in the US.
It seems odd that this event occurred, but especially at a time when that treaty is being pushed. I am deeply suspicious of tragic/terrorist events which occur with optimal timing for political advantage, especially by those who would disarm the innocent.
You are right about the suspicious timing. There have been mass shootings in this country that in at least one case took place the day before congresscritters were to vote on anti-gun legislation.