Ok max force comes at about the 9 o' clock position for a road bike. See how moving the seat forwards or backwards changes the angle the knee is at. Now if you rotate the best fit 90 degrees counterclockwise (now max force at about 12 o’ clock keeping the same knee angle), without a backrest a real hard push on the pedal would throw him off the back of the bike. So what happens is people screw with that knee angle causing problems.

Nice drawing eh?