My thinking is the same as yours. Beheading reeks of AlQuaida influence. I, too, wonder how many of them are here and in Mexico. It wouldn’t be much of a stretch for them to set up shop there.
Lots of societies have indulged in decapitation, notably including the Irish and the French.
Here’s a discussion of the possible history of the method in Mexican drug wars.
http://www.time.com/time/world/article/0,8599,1839576,00.html
Apparently rare prior to 2006. One possible explanation is that the thugs saw the Islamist videos and thought, “Cool!” Another is influence from MS-13 and other Central American gangs.