Knowingly sold firearms and ammunition to individuals that were felons and or under indictment and could have been imprisoned for more than a year(even if they weren't.)
Strike one
In possession of unregistered NFA items.
Strike two
Multiple counts of the above.
Strike three
Game over.

With him being open for a short while makes me think the alphabet crew were tipped off in regards to questionable practices.

Almost every instance where I have heard where the Feds have stomped on a FFL it could have been prevented if the FFL was on the up and up.
As noted above: play stupid games.