Julia
What are your rear shocks adjusted to? Do they have bump stops?
If too hard and no bump stops the shock absorber is transmitting the forces from the road springs directly onto that mounting, and this might have contributed to the failure. However I do suspect Peter is correct and that it is a fatigue failure.
I have personally broken the bottom mounting plate, but never seen the top on ego like that, although with you and Richard both having a failure there it is obviously another weaker point.
