There is really only 2 reasons to why excessive oil is pumped out of the breather. One is too high crankcase pressure and the other is too high oil level.
Too high crankcase pressure can have a few causes, by either restricted ventilation or something wrong with the pistons, rings or lining.
I can only guess, based on your info. Most probably the first time you felt the engine slow down or run laboriously, the piston rings got fried and lost tension. It could be caused by a temporary overheat from a number of reasons, Obvious is a lack of oil but also too much oil is also a reason that could cause the engine fry because oil is slow to shear heat.
Other cause for overheating is slow running, lugging, ignition timing wrong, lean carburator adjustment, high speed! With fried rings from there it usually is just a slip-slippery-slope downwards with more and more blowby, when combusted gasses gets past the rings. Those hot gasses elevates the temperature in the oil locally -and in general- to the level that the oil can break down. The amount of oil splashing around and churned in the crankcase is very small, so it gets overheated easily.
With too much oil, the heat transfer from the oil to the air is impaired and heat is building up together with the brake of the spinning crankshaft that the oil makes, makes the engine labour more than normal and it all results in overheating. Oil is very slow to shear heat so it needs to be churned to a mist in order for the small drops to get in touch with as much air as possible to cool off.
Yeah.. I know...It's a wonder that it work at all!
