ACI may be caused by inadequate filtering (such as incomplete filtering of unwanted modulation products in FM systems), improper tuning or poor frequency control (in the reference channel, the interfering channel or both).
B emitting power into A's channel is called adjacent-channel leakage (unwanted emissions).
First, because RF filters require a roll-off, and do not eliminate a signal completely.
Second, due to intermodulation in B's amplifiers, which cause the transmitted spectrum to spread beyond what was intended.
Similarly, B's signal suffers intermodulation distortion passing through A's RF input amplifiers, leaking more power into adjacent frequencies.