My guess is that you have a defect/hole in your inner tube. also could be that the presta valve is not closed all the way (assuming you have presta). If you've screwed the valve down proper and the tire still goes flat, take the tire off and inspect it inside and out for embedded stuff. All it takes is a staple, really. Inflate the inner tube and listen/feel for escaping air. Re-inspect the rimtape and rim before you put the tire back on.

no way could any extra pressure you could put on the back tire cause a flat all by itself (i.e.: panniers) if the tire is inflated up to spec.