Hello,
I am rather new at this myself so take this with a grain of salt. Looking from the Patch to the port, the length of microstrip line is acting as a loaded t-line, with the port impedance as the termination. As the length changes, so does the impedance it presents to the patch. At some lengths the loaded line will present a somewhat better match to the patch impedance causing the return loss to drop.
Hope that helps, maybe someone more experienced can chime in.
-Sami