The reliability of Si photonics and optoelectronics devices is emerging as a major new topic. By using TCAD simulations, this work investigates the microscopic origins of the Ge High Speed Photodetector (HSPD) performance losses during stress obtained in [1]. It confirms the key roles of the carrier lifetime degradation on both dark current increase and photonics current decrease, which could be triggered by surface recombination (SR), especially at the Buried Oxide (BOX). Other sources of degradation are studied, as fixed charges in the SiO2 passivation layer and interface state (Dit).