1) Killing key encounter mechanics (nails, certain adds, etc.)
2) Being brought under 15% hp and surviving.
3) Healing a player under 15% hp.
4) Interrupting
5) Attacking
6) Healing
7) Being attacked, and surviving, while under 15% HP (HUGE increase here)
8) Dispelling debuffs

Some add more than others. Some trigger an "effect" which causes the bar to periodically tick up. Some simply add huge chunks (IE #7).

Example: Have a paladin pull a ton of mobs, get to 10% hp, and bubble. Every swing will add a large portion of the bar (You can cap out a limit 3 in seconds this way).