What Causes Loss Of Balance While Riding The Bike?

I am a reasonably fit 67 year old male who has, over the past 12 months or so, fallen of different motorcycles 5 times while reversing the bike by pushing with my legs. My feet are well planted on the ground and I have no sensation of the bike becoming unbalanced until it is at an angle where I am unable to hold it up. I often ride and have no problems with handling the motorcycles at low speed or high speed. At first I ignored it but after last saturday and my fifth fall I am beginning to get worried.

Cheers.