Because it is a line effect
Area 60-ft. line
It must originate from your postion.
As far as being tiny and using it upward, there are no specific rules for that, so it is going to depend on the GM. So that will vary from not allowed, to works just fine.
Personally I'd allow it, but reduce the distance "pushed" by half (or maybe even quarter - similar to a vertical jump). So you'd need a pretty high CMB roll to actually get them into the air with it.
A line-shaped spell shoots away from you in a line in the direction you designate. It starts from any corner of your square and extends to the limit of its range or until it strikes a barrier that blocks line of effect. A line-shaped spell affects all creatures in squares through which the line passes.
So you can totally push something up . . . if you can draw a line from one corner of your square upward to the thing in question. Have fun knocking around passing seagulls, but watch for, ah, splash damage.
Edit: This could be pretty interesting if you have a burrow speed and can make your way underneath your opponent.
Could be effective, but watch out for the size limit on bull rush. never mind, hydraulic torrent has an exception.
BTW, you do know what 'knock up' is slang for? At least in Kiwi English.
In American English slang "to get knocked up" is to become pregnant. Just to clear that up for anyone.
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Returning to the opening question, I am always wary of giving spell extra power based on a lenient reading of them.
Hydraulic torrent makes a Bull rush, and a bull rush can't force a creature to fall prone.
But if you push someone in the air when it falls it will become prone.
Then there is the problem of where it falls. In theory, it falls back in the same square as you are pushing it directly above you. Do you take damage from the falling creature?
Then there is the simple limit that the Bull rush maneuver doesn't push someone in the air.
Lastly, there is one spell that does that (plus some fire damage) Geyser. It is two levels higher (unless you are a druid), so you are getting what I would consider the most important benefit of the higher-level spell (getting you opponent prone) with a noticeably lower level spell.
All included, I laud the creativity but I will not allow it in my games.
It is a fun effect and in a game less structured than Pathfinder or where being prone near a melee combatant isn't an almost guaranteed death sentence I would allow it, but not in Pathfinder.
Strictly RAW Bull rush can't push someone in the air.