This occurs because the film thickness is exactly half the wavelength of the wave in the film. One final philosophical note, to really make your head spin if it isn't already. In the diagram above we drew the two reflected waves and saw how they cancelled out. This means none of the wave energy is reflected back into the first medium. So, even though we did the analysis by drawing the waves reflecting back, in some sense they really don't reflect back at all, because all the light ends up in medium 3.
Destructive interference is exploited in making non-reflective coatings for lenses.
The coating material generally has an index of refraction less than that of glass, so both reflected waves have a shift.