Why can't my dog move when he's asleep?

okay this question isn't is stupid as it sounds but i figured this particular phrasing of the title would get alot of attention which is what i need cuz i can't find any reliable info. my dog (a min. schnauzer) was sleeping then out of nowhere he jumped (immediately from laying) off the couch, landed perfectly on the floor & stood with his head down swaying it a little & walking very slowly backwards (if you've ever seen a dog bump its head or nose too hard--it was like that) & couldn't un-stiffen his muscles for about 30 seconds. then he was happy & playful (which is why i'm reluctant to accept the possibility of a seizure but maybe i'm lying to myself). about a year later (he's only two) he did something similar, only this time for about two minutes & this one seemed more like a seizure. his muscles stiffened to where, if i stood him up, he'd fall right over. his eyes were wide open though he didn't seem aware of anything (i have no way to be sure & this hasn't happened again so far for me to check, so this particular detail may be inaccurate) & he'd make very quiet grunting sounds. then he just got up & looked at me like "uh...you're not petting me because...?" i'm not entirely sure if these two events are connected but they don't have any seizure-related symptoms besides exactly what i've described here. it's more like involuntary stretching, intense enough to prevent movement. please help & if you don't know the answer, i don't need (or want) you to tell me that. thanks.