|The Abbey in the Oakwood.|
|Ruins of Eldena near Greifswald.|
As for their demise, there's a number of possibilities. Perhaps they were destroyed by an enemy force, demonic or otherwise. Maybe the paladins formed the backbone of their kingdom's military and so they were attacked first in order to take them out of the equation. The enemy was successful and the buildings were left to nature to reclaim. Vikings were rather infamous for attacking these places because of the amount of loot they could plunder, so that's an idea.
A good idea is to look to history for some inspiration. The religious wars that took place in Europe during the sixteenth and seventeenth centuries are a great example. The paladins or other religious orders get caught in the middle of a war between two versions of the same religion, much like the wars between protestants and Catholics. In England, at around the same time, Henry VIII ordered the dissolution of all monasteries and abbeys in England, Wales, and Ireland when he established the Church of England.
Regardless, you could certainly use abandoned abbeys and monasteries as either a questing point or a nice little diversion from your campaign's main quest. There's bound to be items left behind in the decaying structures, things that were never looted or scavenged. Maybe the paladin or cleric in your party stumbles upon a holy relic or a new tome of spells or the like. A possible quest point could be to locate and bury the remains of the people who lived there and were killed by whatever disaster befell their home.
I'd imagine if your party were to come across any hostiles, it would probably be bandits or wild animals. They could also be mistaken for grave robbers by a passing patrol of soldiers.