Quick: Normally when a seal does this it is because the bearing is worn out . The seal then is stressed and pushed out since it is designed to retain grease or oil not stabilize the shaft from radial or lateral run out .
I have seen this also if a engine has been overheated and the seal become distorted , but this is an extreme scenario . Very seldom is it a case of a worn out seal only , but it does occur , check your bearing condition 1st then if ok perhaps just replace the seal .

on a car engine the rear main seal gets hammered out A thrust bearing goes and crank shaft end play wipes out the seal. Sitting is also bad for seals, I would drive it back in and keep using it whilst keeping an eye on the oil level.

Is it leaking oil there or anywhere else? I have never seen a mower engine with a seal that pops out except 2-cycles, and that's because they normally have compression in the crankcase every cycle. Plenty of 4-cycle seals will leak, and they're easy to replace. The tedious part is cleaning/polishing the exposed shaft to prevent damaging the new seal on installation. Like said above, re-seat it if it's not leaking and replace it if it is. On some engines it's possible to 'stake' the seal in place. This is taking a pointed tool with perhaps a small chisel shape and deforming the surrounding metal in three or four places to prevent it from popping out. This also makes it harder to remove next time but might be the answer for this particular model. However, with OPE engines, there probably will not be a 'next time' for this repair. I've only had to replace one 4-cycle lower seal one time, and I've never had another one to go bad. Probably because I'm an oil and oil change fanatic.

"Probably because I'm an Oil and oil change fanatic."

More likely because your a dedicated and cautious individual , who does not inadvertently try to mow Rock Gardens or remove Stumps with your Grass Mower lol :)

Thanks Guys,

Those are helpful ideas. I'd not considered the bearing runout as a reason for the seal loosening as I'd never had one leak or move ever before. I'm very careful about my oil changes and so I'm not concerned about abuse. I did put the seal back into position and even made a spacer ring to preclude it's movement in the near future. (It needs to last until only next week when the leaves are all removed) The oil loss is really bad now, leaking about 1/2 capacity and hour, so re-installing the seal was too little too late I'm afraid.

I shall check the bearing and then if it's okay, get a seal replacement. Thanks for the great ideas.

QR

A quick homemade seal driver is a piece of PVC pipe just big enough to slip over the shaft and cut as square as you can make it on the business end.

Good Luck Quick , you most likely will have no issues for a short 1or 2 week duration . As Sax has indicated new seals are rather inexpensive .

So today I washed the motor down and tried to install the seal I'd ordered only to find the sent one too small. The good news is that the shaft and bearing are in fine shape and not worn out or loose. I expect the seal replacement will fix the problem. ( I saw a large chunk of the seal broken off as if it was due to rubber hardening etc.)
QR

QR: Great to hear that the bearing appears ok . Normal Ageing causes the Hardening of Seals normally due to the excessive Thermal Heat Cycles that they endure. Just a word of info you may wish to use a Higher Milage Engine Oil Type when you replace your oil after the seal replacement . Valvoline or Quaker State have multi Blend and Straight 30 High Milage Products that have Seal Conditioners within the Blend to help with Seal Preservation . Otherwise there are other Lucas or other Manufactured Products that will help. They are relatively cheap and a little goes a long ways between oil changes . All The Best in the New Year Dude !

Guys,

So I got the correct seal, it slid in tightly enough, I went ahead and did the oil, gas filter, plug and air filter. I washed the back of the motor where I couldn't reach for years and then ran the motor 20 minutes without a hitch. I believe that he is ready to run for years to come.

Thansk for the ideas,

QuickRick

Good to hear Bro ! all the best !

It was suggested that I check the rings for blow-by and the ventialtion system for cleanliness as a matter of peace of mind. (it never smokes or uses much oil)
I'm unsure of how many pound psig the compression should be, but I can probably find that where I saw the side mounted breather system.

QR