If the drivetrain is worn and this is just now beginning to happen some form of chainsuck is a likely cause, especially it you put lots of miles on the middle ring.

The other possibility (especially with Shimano derailleurs) is a mis-adjusted B screw which is allowing the upper pulley engage the sprockets through the chain. Buried in the last step of the Shimano instructions is a reminder to check pulley clearance in all gear combinations. This is because upper pulley height varies with changes in chainring size.

If the problem is uniform regardless of which rear sprocket then the chainring is the common denominator and some kind of chainsuck is more likely. If it's worse on larger rear sprockets then check the upper pulley clearance.