OK, it sounds like it dropped to the outside with the chain coming from the outer side of the cassette. This is a combination where the chain shouldn't derail, as the line is normally pretty decent.

Take a few minutes to make a few simple measurements. You'll need a friend, a straight edge about 26" long or so, and a smaller ruler. Shift the bike to a gear similar to when it happened. Have your friend hold the straightedge vertical against the rear wheel, just forward of the derailleur. Measure the horizontal distance from the chain to the straightedge, on both the upper and lower loop.

I'll bet 2 beers (imported) or a bottle of Chain-L that the lower loop is farther out than the upper.

BTW- I should add that there other possibilities, chain wear, chain lube, chainring wear or tooth profile, but start with a measurement because it's free and will conclusively confirm or disprove that the RD alignment is a likely factor. If it is have the hanger checked and see if that solves it, if it isn't derailleur alignment, then you can move on to the other possibilities.