its loose
Not necessarily. If the oil filler cap wasn't on tight it could have just vibrated off. If the cap is not on the filler tube it will blow oil out of the tube with a good running engine. I would refill the oil, tighten the cap, clean up the oil spill and try the mower again to see what happens. Chances are everything will be fine.
high milage blow by or pcv Issue
Neither your oil filter, or the fill cap need to be loose, the filter should be screwed on as tight as you can get it by hand only, and the fill cap should fit snugly so it does not come off. If the oil filter is loose or too tight you can blow out the seal which will cause you lose all your oil.
It can cause your oil cap to blow right off, foam around the oil cap is more likely to be water or coolant. Get it checked right away because it can damage your engine. It's usually a sign of a bad head gasket, it lets water into the oil and oil into the water. Check your radiator to see if there is oil in the water and if the water is low.
check pcv it maybe plugged
The engine turns off when the oil cap is removed because the engine is designed to do so. When the oil cap is removed a vacuum leak is created which causes the engine to turn off.
Take the oil cap off of engine and pour in oil.
Excessive blow-by (a tired engine) and/or the PCV system is not functioning correctly. Also, check the condition of the gasket under the oil fill cap.
there is a check valve in the oil system that is stuck open over filling the oil tank will cause the dipstick to blow out of oiltank
No , not for the engine oil cap , just turn it by hand
Look in the link I will make blow.