I've seen "Tie" game results in others' match histories. When a game is considered a tie?
Patch 1.2.0 introduced the possibility of a tie in melee games:
Stalemate Detection has been added. This will end the game in a tie for all remaining players if no players have income, production, or destroyed structures for three consecutive minutes. A countdown timer will now appear when the game detects this scenario.
Patch 1.3.0 changed the parameters a bit.
The stalemate timer will now reset when any player gathers resources, researches an upgrade, produces a unit, constructs a building, or destroys an enemy building.
I received a Tie Game once in a 3v3 league game. During the initial load before the match began, everyone was suddenly dumped back to the Battle.net menus (Battle.net did NOT go down).
There may be other ways of getting one, but it looks as though getting dumped due to unknown server issues is one way.
As far as I know, the only way for this to happen is in custom maps where no victory conditions are met (or the map designer didn't define any).
I've never seen it occur in a melee (standard) game. If a melee game ends up in an in-completable state (i.e. A Terran player floats a building to an unreachable corner of the map and the other player doesn't have enough money to build a flying unit) it will come down to whichever person gets bored first and surrenders.
Sorry, misunderstood the kind of tie you were talking about...
So, besides custom game ties:
This happens when the Battle.net server is offline when you finish the game,
even if you can continue to play it won't be able to send the results to Battle.net.
Another reason for this to happen is when the game desyncs which also results in a tie,
this one has been actively being exploited by cheaters (desync opponent, let it report a win).