What determines bonus xp?

Sometimes when I kill an enemy, I get a popup saying that I received bonus experience for the kill.

Why do I get this bonus XP? I can't seem to narrow it down the variables. It happens whether I'm partying or solo, and whether I'm in an event or just grinding regular mobs.

"Bonus Experience - When you kill an enemy in the world, you might see 5 xp + 2 xp pop up . What this means is you’ll gain bonus experience depending on how long an enemy has been alive in the world, which gives you even more incentive to explore."

Enemies that have been alive (bear with me, that's a huge article. Somewhere halfway through) for a longer amount of time than other mobs give bonus experience based on the amount of time they've been around. This is to discourage grinding in one place.

Note: The amount of XP you receive is also respective of your level.