Why did Kyurem's HP keep increasing?

Question

This one time I was training my Kyurem (who is now level 100) in Mistralton Cave. Then after a wild Pokemon battle... KYUREM'S MAX HP INCREASED! I haven't a clue about what happened. So, a few times, a battle was engaged, and after my Kyurem beat it... POOF. Kyurem's HP maximum went up by 1! Why did it happen?

(This may give you an idea-Kyurem held a Power Weight.)

Answer 1

Your kyurem was gaining HP ev's. The HP ev's were increasing even higher becauses you had the power weight. The power weight promotes HP gain on level up, but halves Speed stat in battle. It adds 4 HP EVs whenever experience is gained in battle, regardless of the Pokémon fought. Its effect is doubled by Pokérus. So if it is at level 100, every four EV's you gained its max HP will go up by one. So what happened is that your kyurem was being EV trained in HP.
Source: http://bulbapedia.bulbagarden.net/wiki/Power_Weight#Power_Weight