I believe the thing that causes us to fall is our failure to really believe God is who He says He is, and will do what He says He will do.
In other words we fall from grace when we trust our own efforts instead of His. When we think He doesn't really mean what He says.
We say things like "keep the faith," or remind ourselves that Jesus' worst condemnations were of "little faith" or failing faith, or simply a lack of faith, but then we totally forget to preach about the necessity of, and blessing from, confident trust (faith) in Christ.
Faith is THE Pentecostal message. In fact Peter spent all of
Acts 2 attempting to convince his audience to have faith in Christ. And only when faith arrived (they were pricked in their hearts) did he implore them to repent and be baptized.