I don't know, when it came up in a 4e game, we decided that prone applied to a Gelatinous Cube just meant it was flipped onto it's side. There's no reason to expect all faces of the cube can propel it forward- since it has no ability to stick to walls or ceilings, it may very well have an "upright" position. And tripping isn't the only way you can knock something off-balance.
You might expect that a cube would be hard to topple, but for all we know, a gelatinous cube might not weigh very much or be top-heavy. There's always explanations for things that "don't make sense", the issue, I've found is, unless there's a canon explanation written down, people create their own in their heads and seem to not want to accept an alternative.
Prone has been a funny condition since it's inception; it says it means being knocked on the ground, but really designers use it as an "unstable of off-balance" condition. Hence why you can (sometimes) knock flying or swimming creatures prone and make them fall, or (sometimes) knock creatures prone without legs.
I remember Pathfinder 1e being a bit confused about this; you couldn't knock flying creatures prone, and some creatures with tons of legs or no legs couldn't be tripped (like say, Merfolk)...but not all of them.
As an aside, I remember a funny 3.x discussion about the difference between going prone and kneeling being extremely fuzzy, and it being a move-equivalent to stand up from either state.
Rules aren't always going to line up with fluff, nor are they meant to (unless you're, say, playing GURPS). Gary Gygax himself says as much in the 1e DMG that the rules are a necessary abstraction of what is really occurring in the game world. Lots of things happen in the normal course of play that don't make much sense. Or, to paraphrase MST3K:
If you wonder how dragons fly or breath fire, or other science facts (la, la, la, la), then repeat to yourself "it's just a game, I should really just relax".