Synchronizer, usually.
Does it pop out when you back off the gas?
It can be bent or loose shift forks too, but then it's usually stiff and/or sloppy at the shifter.
If one of these tranmissions pops out in fourth, it's usually from the driver resting his hand on the shifter while cruising down the road.
With popping out in 2nd and 3rd, it's mostly just wear and tear.