Aha!

I discovered something very important about my Hobie 16 yesterday.  The mast was full of water.!

A mast full of water is much heavier than an empty one and that extra weight was as much as 28 feet above the water.   Could that have been what made me capsize so easily?   The answer is, "Certainly yes, provided that the water was there before capsizing."

The bad news, is that I can't be certain if the water was in there when I bought the Hobie, or if it came in while the boat was capsized. 

• Libby and I were unfamiliar with the "normal" weight of a Hobie mast.  
• I did look the mast over and I did apply some sealants before sailing to keep water out.  That applies specially to the mast head where I had replaced the sheaves.  There's a water barrier there, and I put sealant on it.
• When the mast was up, there was no sign of water dripping out from the internals.

So I guess, I'll never know for sure.    In any event, I drained all the water out, and I took extra care to inspect and re-seal every screw or rivet hole where water could get in. 

Before sailing next time, I propose to put the mast in the lake to float by itself.  If it sinks, or if I hear water sloshing inside, I'll know there's a leak.