Never seen a cleat fail in that manner, but would suspect (as does OP) short lines. As a fellow multihuller I would recommend having another look at Thinwater's posts #18 and #24. We're a tri, so a little different, we lead our springs to cleats on the float/ama, but our bow and stern lines always go to the main hull
(and thus are always at least 10'/3m long).
With cats and tris we have very little curve to the hull
, and you just can't get any horizontal pull when using the near side cleats for bow and stern. Using the outside hull (where space permits) greatly reduces the angle and loading on the cleats, and can (if you like) actually allow you to tie the boat up closer to the dock (since you now have a line that gives largely horizontal pull). At our home dock we actually extend past the end of the finger and (with the dockmaster's blessing) our bow line of necessity has to be tied to the float/ama (piling in the way of tying up to the main hull) so our bow line runs from the ama to the far side
of the dock (since this is the very end of the finger and there is a piling there no on wants to walk there anyway). Again, this gives a horizontal pull which allows us to keep the boat where we want in terms of distance off the dock, while allowing much more stretch and movement.