Input needed -- tying up?

[guest@endurance.net]

Cygnet leva@firefox.org
Need some input here; my mare is definitely not an endurance horse by any
stretch of the imagination, but there's
probably more wise and experienced horse people concentrated on this list
than anywhere else I've ever seen.

23 year old mare. Was off for six months with a ruptured tendon sheath.
(Blew it out kicking a stall, but that's another
story. Thank goodness I don't have to board her anymore.) Started her
slowly back into work in June.
Just been riding on firm, graded, flat dirt roads, 2-3 times a week.

After a 45 minute trail ride on flat graded dirt roads mostly at a walk
(with some jigging and spooking that wasn't out of character for
her thrown in -- she's a hot horse and isn't really happy about all the
walking), maybe 2.5 miles total, she tied up -- was in distress within a
minute of my dismounting, though she showed
NO symptoms beforehand. Pulse was 42 when I rode into the yard, body temp
was normal, she dropped to somewhere between
36 and 40 within five minutes.

Wasn't a bad episode,, but I'm trying to
figure out why it happened in the first place. I definitely didn't
override her.
It was a warm day, but I've ridden in a heck of a lot hotter. Her body
temp was normal and she wasn't
dehydrated.

Mare's 23, drinks about 15 gallons of water a day, shares about 30 pounds
of alfalfa a day with two very large goats,
gets a scoop of red bran every night for sand colic prevention  -- but
hadn't had any in several days because I'm out --
is in apparent good health and is turned out in a dry lot 24/7. No grain.
My well water is a little  high in (table) salt, also,
 though it's quite potable and she's been drinking it with no ill effects
for six months.

She's an arab-x and best guess for cross is quarterhorse or appy. No
papers.  Apparent good health, no signs of cushings
at all.

Input anyone? I'm baffled.