Huh...
won't comment on how you could've avoided that by not double jumping that early ( its a bad habit that i'm working on too 0,o ) nor will i go on about how the tether recovery really is that bad. I don't even have any real logical reason or understanding to explain it... I mean theres no denying even that the chain length is far less than how far down you were, even overestimating how much vertical movement UpB gives olimar himself.
Closest thing to an idea of how this happened is I imagine you were attempting to UpB right as you were spiked, to either hit him or grab the ledge? and that may have buffered due to the end of your air dodge, somehow coming off at some random single frame lacking hit stun despite the stun prior and what should be after that one frame you somehow caught. The chain animation didn't show due to some glitch or something, but once the chain time wise would've grabbed the ledge it put you there.
really... Just huh...
-True