The defroster uses the air conditioner compressor to condense moisture out of the air. On a cold winter day, if you blow warm moist air onto cold glass, the moisture will just condense out, fogging the glass. The defroster blows air first over the A/C coils, to condense out the moisture, then over the heat exchanger, to warm it back up. I won't say that the A/C compressor always runs when the defroster is on (it is probably not needed on really cold days for complex reasons) but the compressor certainly runs on warm days.
So what you were doing was running the A/C, and blowing the cold air directly on the window glass instead of through the face or floor vents. This certainly could crack the glass, because a strong temperature gradient (like, the glass is 100 degrees on the outside due to the air temp and the hot sun, and you are blowing 40 degree air on the inside) causes a lot of stress on the glass.
So, yes, you could have cracked the glass by doing this. But there is no way to prove it with certainty (other than examining the glass with an electron microscope).