Flat plate supersonic flow: trailing edge flow angle

My questions is the following:

Consider a supersonic flow over a flat plate at angle of attack. The flow direction downstream of the trailing edge of the plate, behind the trailing edge shock and expansion waves, is not precisely in the free stream direction. Why?

Apparently there's an effect that causes the flow not to 'shock back' to exactly the freestream direction.

I hope somebody can help me.

Thanks in advance!