If you have only partially vape ejuice remaining in the coil after so long it gets gummy and will get the burnt taste going. Over time that will happen anyways and why coils need to be soaked every so often depending on how much use they see. When your battery or mod's battery is low the process starts a little sooner however and why the green led becomes amber/yellow before red to let you know it's time to recharge.
That is yet another good use of my sonic cleaner. I have kanger coils that would normally have been replaced a long time ago, that still chuck flavor and clouds. I tend to run my tanks all empty, and the night before, I refill, I disassemble rinse, and place everything in a sonic cleaner for several cycles. I let the parts dry on a clean towel over night, and reassemble, refill, revape.
I am fortunate, I have enough RBA's and mech mods, I do not miss a couple tanks for the course of an evening.