Hmm... The end mortises failed... I have no idea if this is a valid
point, but it seems to me that there's a strip only 3/8 or so thick
holding
the outer frame to the inner slats of the seat - that is, all the stress
is
concentrated on the upper face of the mortises when you sit on the edge of
the bench.

SNIP

I will second this. These were my thoughts when I saw the picture as well.
Although weather contributed to the bench's demise, it was most likely a
design flaw that resulted in the failure.

Switching the direction the tenons run (vertical instead of horizontal)
might help.