GPS multipath is caused by the reception of signals arrived not only directly from satellites, but also reflected or diffracted from the local objects. Multipath results in an error in pseudo range measurements and thus affects the positioning accuracy, since the multipath signal takes a longer path than the direct signal.
Multipath results when the direct path to your receiver is blocked (by your body, your house, roof, trees, mountains, buildings, etc) and the signal from the satellite is REFLECTED by some object. The reflecting surface may be: buildings, mountains, the ground, or any object that happens to be a radio reflector at 1.6Ghz.
So we can conclude that multipath error is Caused due to poor sitting on the receiver antenna.