If the outside antenna for a cellular booster picks up the signal from the inside antenna instead of the cellular tower, oscillation happens. If the booster is properly designed, it will power down to prevent this - thus limiting the effectiveness of a booster.
This is the same phenomena as walking too close to a speaker with a microphone – leading to a howling screech. It's a feedback loop.
The best way to avoid oscillation is to put as much distance as possible between the inside and outside booster antennas, and to keep the antennas pointed away from each other.