It is a question of avalanche multiplication process which provides an internal gain. An incoming photon produces an electron-hole pair that are accelerated in opposite direction by the electric field. These carriers may collide and ionize atoms, producing other electron-hbole pair. Below the breakdown voltage, the avalanche dies after a few nanoseconds. A space charge charge appears and reduces the effective electric field leading to avalanche extinction. Puse heigth depends on mean optical power received on diode. above the braekdown voltage, the electric field is sufficiently high to overcome the sapce charge effect. Triggered, the avalanche grows exponentially until being limited by the diode resistance (low valu si high current!).If not quenched the current flows through the diode until its detruction by heating.
Below breakdown, I think that the leakage current increases with the reverse voltage. See datasheets for more precise information on incraesing rate and absolute value.
I hope i'll lep you.