Some good advice, some not quite claims and some missing advice.

Yes, the crank arm needs to be properly torqued down in the beginning. No, it only takes one person with proper technique to do this (placing the arm and wrench close together so when tightening the wrench/bolt one's hands are pressing together, and a wrench with enough lever arm). What's missing is that the arm's retaining bolt should be checked for proper tightness after a few hundred miles. This is but one of the reasons that better shops offer a 30-90 day free bike check up/tuning after the sale. Failure to do so can result in the posted problem (as well as other "I was just riding along..." problems. FAILURE TO DO PROPER MAINTANANCE ISN"T THE SHOP"S FAULT. (not that the OP said it was).

I'll bet a good beer that it was the LH arm that came loose. It almost always is. All of the stress of pedaling (and half of the body weight) is going through the fairly tiny contact surface between the arm and the axle (which with a tapered square design is less then a square inch). The RH side loading is less because at least half the pedaling force is going directly through the crank arm and into the rings. Andy.