On the majority of British Classic Motorcycles the lubrication system is of the dry sump type and is operated by a double gear pump. The oil is stored in a separate tank and drawn into the engine by the oil pump when the engine is running. The used oil is then pumped back into the tank leaving the sump dry. To achieve this the return pump has a greater flow rate than the delivery pump which often results in it sucking up air bubbles into the return oil but is not a cause for concern. To stop the oil from draining into the engine when it is not running there is a valve (A) in the supply line. If this valve is worn or faulty the oil will leak past and fill the sump when ever the engine is not run for a while.  

After standing 2-3 days there will be a sufficient quantity of oil in the sump so that on starting the engine, the oil in the sump will come out through the least path of resistance and will deposit itself in a large pool on the floor. It can also cause a hydraulic lock which will make it impossible to turn over the engine on the kick-start, as the piston tries to compress the liquid on it's down stroke. In less severe cases the oil will pump back up to the tank when the engine is run, but if you have check your oil and topped-up before you start the engine, you will find that the returned oil will flow out of your over-flow pipe or filler cap and create a nice pool of oil on the floor.

The good news is that it looks a lot worse than it is. 
If ridden regularly, the problem is not noticed, as the oil gets pumped back into the tank.
But with bikes that are stood for weeks, it becomes very noticeable, oil deposited all over the garage floor.