The problem appears to be that ddb watchpoints in the kernel trigger on
   *any* modification to the entire page, not just the given address.  So
   the write to an unrelated address was triggering the watchpoint.

which is what simon told you in his previous mail :-)

   > I don't think I've ever used ddb watchpoints, only hardware watchpoints.
   > It appears at a glance that if you set a watch point in ddb then the
   > entire page is marked readonly so any store in the page will trap - see
   > the pmap_protect call in ddb/db_watch.c:db_set_watchpoints().