There is a possibility of a couple of things ...
Rear cylinder starting to leak and wetting shoe, which can cause shoe to grab on.
Dust and shoe wear in general
Retaining spring broken, or shoe adjusters getting seized.
None are serious. Shoes usually only lock wheel when wheel travels in opposite direction to norm. This is because of the leading edges of the shoe lining can help grabbing on.
If it is not grabbing on forward, get it sorted asap. You may need shoes, cylinders, and a cleanout. Try driving about on some bumpy stuff and warm brakes up a bit.