Waveguide excitation through coaxial antenna
My waveguide knowledge is a bit sparse.  But here's the way I think it works.

What you've shown on the left of your drawing is a classic quarter wave stub. This creates a 180 deg phase shift (90 deg out, 90 deg back) in the signal that's returned to the "T" junction.

But, the waveguide section on the left is terminated in a short.  When the RF reflects off that short, another 180 deg shift is introduced.  So what's returned to the "T" junction has been shifted 180 deg (out and back) + 180 deg (reflection off yhe short), or 360 deg.  That is to say the reflected signal comes back and reinforces the signal propagating to the right, out to the open end of the waveguide.