When the Valar defeated Morgoth, the sea destroyed Beleriand. Was this a result of what the Valar were doing to Morgoth or was it some kind of reaction by Arda to Morgoth's defeat?
It is not exactly specified why Beleriand sunk under water.
Tolkien was a bit contradictory in his story because he specified that the elves and humans fought against Morgoth armies of orcs and balrogs which is obviously not possible if fighting does not occur at land. If the sinking of Beleriand would have been already caused by the War of Wrath as implied by him, the victorious elves and humans would have drowned with it. This is not mentioned so we can assume that the sinking is not a cause of the War of Wrath until the final victory and the necessary time to evacuate Beleriand.
What we know is that the last and desperate defense was an army of flying dragons, with Ancalagon the Black as leader. This one was so big that when he finally fell he crushed Thangorodrim (which are mountains 10 000 m / 30 0000 ft high). So he must be pretty big. (Nasty question: Where did Morgoth store and feed his dragons ? Obviously not in Angband. Never mind). Given that an army of flying dragons was involved, we can assume that every slain dragon flattened down a whole area in Beleriand. Oh, and then there was that the War of Wrath with a duration of fifty years. We can assume Beleriand to be plastered with hundreds of thousands corpses of elves, humans, orcs and balrogs. As both Morgoth and Sauron were not known as masters of sustainable development, we can also assume that under Morgoth reign forests were cleared, fields and mountains poisoned and towns burnt and pillaged.
While not specifically mentioned, I think that given the circumstances the Valar came to the conclusion that the tainted land needs to be cleaned by the sea and therefore sank Beleriand after the remaining survivors reached Ered Luin, the Blue Mountains.