So I've noticed
 

I spend a lot of time in solb killing Lava Duct Crawlers, and noticed that some of the ldc's run at less than 20% and others don't. I came up with a solution by simply hitting FD and standing up because at this point they start running. But my question is why do some run, and some don't? Can anyone help me out on this?

Are your HPs at 18% or less before they get to 20%? If you are low HP aggro before they would flee, they will continue the attack rather than fleeing.

No i could be at 75% and this still happens.

maybe they are le stunzorz for a few seconds and dont run when they would normally?

Haven't experienced this with LDCs. However I am not a melee. When my nuke brings it below 20% and not rooted they always run.

Yeah I can't figure it out for the life of me. Ah well the FD trick seems to get the job done so its all good, thanks guys.

It's likely because someone else is also killing one nearby, maybe even on other side of a wall. Mobs will run later/won't run sometimes if they have a buddy fighting.