It has to do with the center of gravity and the distance to the support points. If you have a speaker with a square foot print with four points of support at the corners, the center of gravity is equidistant to any two points that you tilt against. If you had three points on that speaker (two in the front, one in the center rear) and you tilted against any two points, the center of gravity is still equidistant but closer to those two points than the four point speaker. Meaning, that the three point speaker will topple over sooner than the four point.
A three legged stool is more stable than a four legged stool because a four legged stool only needs three points of support and those three corner points are not equidistant from each other, hence it tilts easier than a three legged stool which has the three legs equidistant from each other. However... it is easier to fall over if you tilt back on a three legged stool because of the center of gravity is closer to the two tilting points. The same tilting distance on a four legged stool will send you safely forward since the center of gravity is further away from the tilting plane.
A long winded way of saying any more than three points of support on a stool makes it easier to tilt back but harder to fall over.
A three legged stool is more stable than a four legged stool because a four legged stool only needs three points of support and those three corner points are not equidistant from each other, hence it tilts easier than a three legged stool which has the three legs equidistant from each other. However... it is easier to fall over if you tilt back on a three legged stool because of the center of gravity is closer to the two tilting points. The same tilting distance on a four legged stool will send you safely forward since the center of gravity is further away from the tilting plane.
A long winded way of saying any more than three points of support on a stool makes it easier to tilt back but harder to fall over.