I think the contributing factors for it capsizing were:
o 35 gal bait tank placed at the very rear of the boat. 35gal
  x 8.3(?)lbs = 290lbs.
o No splash board. Some of the boats/yachts that go by
  can cause a decent wake. 1 if not 2 foot "faces" some
  times.
o Bilge pumps not hooked up to the float
o A little naivete' on my friends part. There were 3 onboard
  with one of them, if he were awake at the time :), being
  aware of not letting it happen.  He got comfy on the
  sofa seat and dozed off. When he woke up he saw his
  brother sitting on the back of the boat with his feet up
  on a cooler because he didn't want his feet to get wet
  due to the water in the boat. :D His reaction was
  something like "Dude, we're taking on water and we
  need to get going(so as to stop the intake of water
  and to give the bilge pumps a chance to pump)!". By
  that time it was too late. The outboard was submerged
  a little and they couldn't get it started. The boat 
  slowly filled up with water and turned over. Oh well,
  at least no one was injured, but they probably lost
  about $1k (or more) of equipment.
Two more pics of my trailer with the small wheels/tires :D