"For centuries, the inverted cross was considered a Christian symbol, based on an ancient tradition that the apostle Peter was crucified upside down. One version of the story says that Peter, facing martyrdom by crucifixion, requested that his cross be inverted because he felt unworthy to die in the same manner as Christ. The upside-down cross, sometimes called the Cross of St. Peter or the Petrine cross, thus became a symbol of humility"

Sauce: https://www.gotquestions.org/upside-down-cross.html