I've learned to adapt to this.
A few speaker blowing thumps will teach you.
My previous digital mixer, Panasonic DA7, did not exhibit this behavior.
Turning it off was silent, even if the monitors were at full volume.
I agree with Jim that the normal procedure for powering down is monitors first.
There must have been some magic in the DA7 and the Yamaha mixers that muted the outputs on power down.
Not a big deal, the Tascam is a great board!