When you have a swarm of voidlings focusing a large monster, it keeps on losing patience. Usually it gets killed fast enough it won't really matter, however this had actually backfired while doing rift herald. I'm not sure if this is a bug, or whether its to do with switching targets, and if it was intentional or not, but I will put this here anyway for you guys to check out.
Thanks 
Its intentional. Switching targets often, among other factors, can cause it to lose Patience.
EDIT: Back with more information. This was introduced in patch 5.1, hopefully this all helps.
Leashing makes sense. More sense.
Over the seasons, leashing went from being a set of basic rules to having so many edge-cases it felt almost random when something would reset or wouldn’t, leaving you without a good sense for what caused it (and more importantly, how to avoid it in the future). With that in mind, we built a new system - called Patience - to better support future jungling patterns and behaviors.
new PATIENCE: Jungle monsters now have a ‘Patience’ characteristic. Monsters no longer reset their aggression based on distance or positioning - Instead, once the monster’s Patience is reduced to zero, the monster will reset and return to their original position. During this reset, they rapidly regenerate health and do not respond to further aggression. LOSING MY PATIENCE Monsters lose Patience under the following conditions:
IT COMES WITH AGE: Large monsters display their Patience in a meter above their heads
IN MY HAPPY PLACE: Once a monster has recovered to 90% health, they regain all of their Patience
YANKING MY CHAIN: Abilities that pull or displace monsters no longer completely reset the monster’s aggro if they already had a previous target
