This has happened to a friend and I both once, respectively. I honestly have no idea what causes it;
After dying we get ported to the respawn area just fine; however, our characters were lying dead on the ground (despite having full health); we were unable to stand up, or move. I could cast abilities (buff/sprint/Smash) just fine; However, Smash didn't show an animation; neither the jump nor the red-flashy lightning-junk.
It's only ever happened on the Huttball map to either of us; so I think it's specific to that map.