Happens with light loads. Primer pops out under pressure like a piston,
pushing case hard forward, in some instances even setting the shoulder
back slightly. Then the pressure isn't enough to drive the case head back
against the bolt, the only thing holding is friction of the front 80% or so of
the case against the chamber wall.
No real harm, but may eventually shorten the brass a good bit.
Primers pop out on all loads, but full pressure loads stretch the case out and
reseat them as the case head is pushed to hit the bolt face.
Bill