The rear end of a bike can slide for a number of reasons. Over-firm suspension dials out compliance and suppleness, making a bike's tyre do more work than the suspension itself.
This can cause the tyre to slide, through a loss of grip. Over-inflated tyres are also more prone to sliding because they have a smaller contact patch than they should.
Most manufacturers play it safe and recommend moderately high tyre pressures to account for a heavy rider and passenger a combination that places great strain on the sidewalls of under-inflated tyres.
Most riders, however, can get away with dropping the pressures a few psi. The rubber will wear out quicker, but you get more grip.