Generally , the junction of P-N diodes works as a barrier. When the reverse voltage is applied then the electric field across the junction increases.
Avalanche breakdown occurs when you apply a high reverse voltage across the diode.
Due to high reverse voltage, the electric field increases and becomes very high.
High electric fields exert a force on the junction electrons and it frees them.
Generated new electrons move with a very high speed and collide with each other.
Due to collision further free electrons are generated.
These electrons start drifting and electron-hole pair recombination occurs across the junction which results in a net current that increases rapidly.
Final answer
Hence, avalanche breakdown is primarily dependent on this phenomenon of collision.