Our boars will do this when they are interested in a sow.  I have never noticed this behaviour towards a pregnant sow.  It is their way of saying 'come on lady I am ready'!  I have seen the boar actually lift the sow into the air to get her attention.  She will only oblige when she is ready though!
It is also a behaviour between castrated males as well, and young gilts towards another gilts and castrated males. It is their way of displaying their urges I guess.
Kate  
 
  