This is the only way I know of, but it edits it for every creature:
Go into Advanced -> Gameplay Constants and edit the following values:
The main reason that corpses are staying standing up like that is because they have no bone animations, normally when a unit dies they decay to bones, which stay up for 88 seconds. Because those units have no bones phase, they glitch and just stand for a while.
You can make these values 0 of you like, just in my map I had them set to 2, so you could still see them when they die for a few seconds.