Short answer: Leading edge thrust.
Slightly longer answer: Subsonic flow has the capacity to negotiate the leading edge, resulting in a suction peak which is missing in supersonic flow. Sweep allows to keep that flow pattern for as long as the flow speed component perpendicular to the leading edge is subsonic.
This suction peak, acting on the forward-facing surface of the leading edge, pulls the wing forward, hence the expression "leading edge thrust". This is missing once the perpendicular speed component exceeds Mach 1, exposing the forward-facing area of the wing to the increased pressure aft of the shock.
This characteristic can be maximized by conical camber, a cambering of the most forward part of the airfoil which increases with span, such that the outer forward wing has a conical surface. This has been introduced with the F-106 and is used on many supersonic designs.
F-106 at an angle to the sun which nicely shows the cambered leading edge (picture source)