I remember the developers saying on their blog that the crits in this game had a system or like, a balanced trigger allowing them to happen. Anyone know what it is?

"There is about a 5% chance for unleashing a critical hit when you fire your weapon at the beginning of the game. The critical hit deals extra damage to the target and the damage dealt is independent of the range at which the projectile was fired from. Regardless of how you rank on the score board or how many players are in the server the chance of scoring a critical hit is the same. The frequency of critical hits one yields is proportional to the amount of multiple kills in quick succession that player achieves. This seems to vary from class to class and weapon to weapon, but usually once a player scores a 'Double Kill' the 5% chance increases to about a 20% chance of striking with a critical hit."

"Critical hits are awarded based on random chance at the time the shot is fired, starting at 5% for most weapons. Melee weapons generally have a baseline chance of 10%. The amount of damage dealt by a player over the last 20 seconds can increase the crit chance, up to a maximum of 20% [2]. This 'super-crit' period can be extended by continued killing."
-TF2wiki
-Extended crit testing

Jesus that doesn't seem balanced that seems like it's just up to luck.

as stupid as it seems, its all very, very balanced.