This happens to EVERY antenna, it's more noticeable in mobile antennas because they are mobile.
Probably the largest effect is caused by driveways that are steel reenforced. Most neophyte operators don't think about the metal under their feet so to speak, and they tune up the antenna perfectly. They pull out into the street, and now the SWR is too high.
Both of these are caused by capacitance, albeit the effects are actually on opposite ends of the antenna. The trees effect the capacitance at the top end, and the steel in the driveway on the lower end, of ground loss if you please. Ground loss, incidentally, is the largest factor a mobile operator has to contend with.