pMOSFET reverse Drain-Source current
MOSFETs have a body diode which will conduct when the MOSFET is "backwards biased": in the case of a PMOS, when the drain-source voltage is greater than a diode drop.

It helps to have a MOSFET symbol which has the body diode included:



This is an inherent "feature" or MOSFETs: in order to make MOSFETs work reliably, they end up with this body diode. In many applications: H-bridges, ideal rectifiers, etc, they come in handy, in others, not so much. However, that is simply part of the fun of using MOSFETs. For more information, see Wikipedia: MOSFET body diode and Digikey: Intrinsic Body Diodes.

So, don't connect your MOSFETs upside-down :)