Till now I always tought it was any direction. Rereading the spell, I can't find it in the description.
So I would say that it is reasonable as a GM to rule it comes from the caster. The spell however would be a lot more fun if it comes from any direction. Remember, if you let your players do it, the villains can also do it.
Yeah, this one is wierd...
I personally feel you SHOULD be able to push in any direction, since there isn´t any ´directionality´ in the spell text itself, but per RAW I think you´re still limited to pushing them straight back. I say that because the spell IS doing a Bull Rush maneuver, without any further wording to suggest their is an exception to Bull Rush´s built in ´straight back´ rule.
Now, one could say that one can push them ´straight back´ from a point to the side, but that just makes the ´straight back´ wording mean absolutely nothing... So I think ´straight back FROM YOU´ should still apply here. Note that Creatures with Reach / Reach Weapons can easily threaten all squares including to the sides of a target, but I don´t belive they have the abilty to push them ´straight back´ from one one of those squares, Bull Rush means straight back from your own position... I don´t see anything in the spell that changes that.
One thing: ´straight back´ doesn´t always mean into 1 square and 1 square only... If ´straight back´ doesn´t correspond exactly to 90/45 angles (most often when reach is involved and/or size category is different) then there can be MULTIPLE squares they can end up in (because angles other than 90/45 can ´jag´ at arbitrary points).