Well, we're assuming they were smugglers. It's either that or they're invaders who badly misunderstand the fundamentals of siege warfare.
If you think we're making all this up (and we will not hold that against you), check out this surveillance video shot by the U.S. National Guard, who were evidently too stricken with delighted whimsy to put down their cameras and enforce the law:
Sure enough, the video shows a few guys in an SUV towing the above catapult up to the border fence and firing a few bales of weed over:
"Let's light one of them on fire, it'll be goddamn hysterical!"