What Causes Leg Weakness Suddenly With Fear Of Falling?

Hi! Sometimes, in the morning only, I suddenly feel weakness in my legs and have to sit down for fear of falling. The weakness does not last long, perhaps 10 seconds or so and it happens seldom (maybe two times/year); so far, perhaps four times all together. Otherwise, I am very healthy, go for 2-hour long walks with my dogs daily, lift weights, my blood pressure and cholesterol is low (while HDH is higher than normal). I am over 60 years old.