The impeller is running backwards. You will notice a slight reduction in head and a larger reduction in capacity. The shaft is running backwards because of a wiring problem. The pump is running backwards because the discharge check valve is not holding and system pressure is causing the reverse rotation. This is a common problem with pumps installed in a parallel configuration.
Check valves are notoriously unreliable.
The impeller has been installed backwards. This can happen with closed impellers on double ended pumps The second stage of a two-stage pump is wired backwards. The pump reverses when the second stage kicks in. You should have heard a loud noise when this happened. The wear ring clearance is too large.