In case part of the transmitter’s (TX) signal reflects onto a big surface (hill, house, water tower etc) then bounces and reaches the receiver (RX) with phase difference. So at the same time the RX receives two signals that confuse it. The model reacts as if submitted to interference from another TX transmitting in the same frequency. Older generation systems are more prone to this phenomenon compared with the modern 2.4 GHZ transmitters and receivers.  

      Hopefully because the model is flying, it leaves fast the spot where the bouncing signal is received and immediately stabilizes itself. This self interference is known as glitch. If such conditions exist, then the phenomenon will occur always at the same spot of the flying field, so keep away, otherwise you may…..