Three potential causes: 1) Either the pressure switch that's supposed to turn it off has failed, OR 2) the pump isn't putting out enough pressure so it just keeps running OR 3) potentially the check valve is stuck open and not holding the pressure (this last one would be evidenced by the pump shutting off and immediately restarting).
Most probably, as with DXSMac's problem, is #2 - the pump failing.
1988 Coleman Sequoia - popup (1987-2007)
2012 Roo19 - hybid (2012-2015)
2016 Mini Lite 2503S - tt (2015 - ???)
2011 Traverse LT, 3.6L, FWD
2009 Silverado Ext Cab, 5.3L, 4x4, 3.73
2016 Silverado 2500HD Dbl Cab, 6.0L 4x4, 4.10