Board Thread:Skyrim/@comment-1157147-20130322022136/@comment-221.234.187.178-20170324172722

It is also possible that both the Giant and the Dragon fell into the hollow (I have mistaken the word to "hallow" in the above post) when the Giant stamped a hole on the ground.

After falling in, they became so angry at the Dwemars for interrupting their fighting game (above the ground) this way, so they massacred all the Dwemars there. After that, the two of them stopping fight each other, and had the whole Blackreach cave for themselves, like, living happily thereafter... until you, the Dragonborn, disturbed the tranquility under there once again...