Rigid paralysis....or as someone once called it "the timber effect".
It happens with MC. It's only happened to me once though. I tend to have more of a propensity to freeze into place somewhat. Another reason I might be less prone to falling is most of my mass is in my lower body so I'm less likely to tip off balance.
Before you try to get up from a fall, try moving around on the ground to warm up and then slowly try to get yourself up. Don't try to rush it.
The straightening out might just be hyperexcitability and you should try to account for that when getting back up. Your muscles might be "spring loaded".
I do tend to launch myself and if i land against some thing my legs and the rest the mucles keep driving . i fall to my weak side i think because i am left handed and my right collar bone has been broke 3 times ...lol..left handed people should not drill metal with the left hand if the bit sticks and you can't release the drill it will will near twist yer arm off...i have hundreds of warnings..like if your driving and your real stiff don't wrap your thumb around the steering wheel...lol
I was using a Dremel a few weeks ago and I thought to myself "That's stupid, this bit rotates such that it causes everything to fly up in my face...they should make it rotate the other way." Then I realized "Oh yeah, I'm left handed." I'm actually ambidextrous but I write and use most tools exclusively with my left hand. With the Dremel, I have to steady it with my right hand but it's still positioned in a left hand dominant fashion.
I discovered that via research that Dremels actually used to rotate the other way, putting right handers in a potentially dangerous situation but too many people complained. I guess I should find an older Dremel. Until then I use a mask and a face shield.
I wear the face shield. I come from a family of scientists and optometrists, it'd be an unforgivable sin not to! And I'd never hear the end of it if someone had to pull something out of my eye.