Then apparently you were far enough below the surface for it to
happen... If you exhale slightly through your nose as you descend,
you'll equalize the pressure in your mask and it shouldn't happen...
It probably never happened for you when you were diving because you
are equalizing your ears and depending upon how you do it, that would
also equalize the pressure in your mask... Then again, since the
airways of the nose and the mouth are connected, just the act of
breathing through your mouth might automatically equalize the mask by
letting air leak back through the nose... Frankly, I've never paid
much attention to it to see if it was possible to get mask squeeze
while breathing through a regulator... Then again, with a mask and
beard, my mask always leaks anyway, so it might be difficult for me to
even get mask sqeeze... I assume that you don't have that problem,
right? <grin>

Impossible.  Mask squeeze is caused by a vacuum forming in your mask as you
go deeper, not by an overpressure.  If you exhale a bit through your nose
you equalize the pressure between the inside of your mask and the external
water pressure.  The only other explanation is your strap is too tight.