Almost collapsing during tacking up...

Yesterday evening I went to the barn for our evening ride. I brought October into the barn, groomed him, and put on his saddle pad and saddle. We go through the same routine every day, and nothing was different. When I went to give him his bit, he was clenching his jaw and did not take his bit as easily as he normally does. I just figured he was being a pain...it had been about a week since we had ridden.
He finally took the bit, and as I was buckling the throat latch his head started to droop like he was falling asleep. I said his name and his head came back up. But then it went down again and his front legs started to buckle...said his name again and back up he came. This happened two more times...all in a matter of about 20 seconds. I walked him out of the barn, and he acted like nothing at all had happened. I lunged him a few times and rode him a little right around the barn and we had no problems at all.
Later the BO informed me that he had been fed right before I got there, and they had a new round bale. Could he have just been stuffed and falling asleep on me? Or do you think this is something more serious?
Thanks in advance!

Hmm, when started reading, thought you were going to say when you were doing the girth up or some such - badly fitting saddles can pinch nerves that can cause horses to fall over. Well worth checking that out - you should check your saddle for fit every few months anyway - & make sure the girth's not too tight, because it could still be to do with that - he may have been in pain & 'zoned out'.

His head nodding could have been just tiredness if he'd had a particularly big day or such, but the knees buckling shouldn't happen. Perhaps there was something pinched that effected his stay aperatus, or perhaps it isn't working for him regardless of tack, which would also explain over tiredness. Does he appear to be able to stand with knees straight, legs 'locked', so as to stand without muscular effort?

Ever done it before? Narcolepsy?

My first suspect when this happens while saddling is generally a pinched nerve that can cause varying degrees of front end paralysis when its pinched by the girth...the cure is to do the cinch up in degrees and walk the horse and stretch his front legs to help alleviate the pinch as the cinch is tightened over about 3 to 5 minutes.

There are other reasons that can cause a horse to do that but this is generally my first thought and is easy to try the fix and see if it still happens.