Facility backpressure effect remains to be an important unsolved problem for Hall thruster development making it difficult to predict the thruster on-orbit operation precisely. Measurements in previous research shows the backpressure effects can cause unexpectedly-high thrust and discharge current increment in comparison with the theory. In this study, backpressure effects were investigated by sensitivity analysis using Particle-In-Cell plasma simulation. Not only static pressure but also dynamic pressure, or the velocity of background gas flow, were parameterized. The results show that if dynamic pressure exists, > 10% increment of thrust and current can be observed even if the recommended pressure criteria for Hall thruster measurement is satisfied. It is thus suggested that the dynamic pressure of background gas could be one of the main reason of the backpressure-effect issue. The primary cause of the thrust/current increment is identified to be the flux of ingested background gas. It is also observed in the simulation that the channel wall volume erosion rate also increases linearly with the ingested flux.