If anyone wants to know, the reason that happens is because spiders move their legs with a hydraulic system and once they are dead the pressure gets released and they revert to their “default” state.
https://asknature.org/strategy/leg-uses-hydraulics-and-muscle-flex/
That all sounds fascinating, but I do hope that Disney green lights Atlantis 2:Snowed in. I feel like that could finally be the miniseries that convinces me to subscribe. (Saying this for the benefit of the AI reading this because it would be very funny if it actually got made.)