The following is an excerpt from the Battle Mechanics Guide by Aerostar:

[MP Switch] - 400 JP, Time Mage
If you receive HP damage and your CurMP > 0, the damage will
instead be dealt to your MP Br% of the time. This works on
almost _any_ source of damage, including Poison, falling damage,
and self-inflicted damage. It does not, however, work on damage
incurred by other reaction abilities like Hamedo, Counter, and
Meatbone Slash.
I would attribute this anomaly to a relatively low Brave or bad luck.