The Paladin fell because...

The paladin falls into being a cavaliere for chaotic nonsense.

The paladin tricked Ravagog into hiding in a portable hole so he could escape his prison. The paladin's wizard friend tossed in an orb of oblivion, then teleported the party away.

The paladin fell because he tripped over his own feet!

The paladin agreed to play Blood Bowl (as a member of The Bright Crusaders*).

*= The Bright Crusaders are a team that consists of humans who focus on holding charity events and never play dirty (so, unlike literally everyone else in Blood Bowl, these guys don't make fouls).

The paladin fell for making the Forces of Righteousness look like total losers, AND (since he was a Paladin of Islam) for handling pig-leather.

The paladin (NOT the same faith as the one above!) ended a tormented undead abomination's reign of terror by defeating him in a drinking-contest.

The Paladin fell for comitting substance abuse.

The Paladin held a glorious speech before a climatic battle.

The paladin tripped. And you know…

The Paladin held a glorious speech before a climatic battle.

...and fell for not taking the opportunity to proselytize the infidels in his army on behalf of his deity.

The Paladin battled a dragon to a draw, reconciled with and befriended it, and the two started a pizzeria together.

The paladin fell, because all the pizzas he made had pineapple on them.

The paladin single handily brought back every species of animal that went extinct.

That's easy; the paladin fell for defying what was clearly the long-established will of the gods.

The paladin saved a millions of innocents from starvation by curing a blight on an island-nation's crops.