Why high pressure on low side of ac?

There could be a number of reasons, and those could be narrowed down a bit if you had included the actual pressure it was running (along with the high side pressure), as well as the ambient air temperature during testing.

Possible causes may include excessive refrigerant and/or oil, a low pressure switch failure which causes the compressor to run constantly, and blockage on the low side. Incorrect testing methods are also a possibility.