Yeah its just the protectors.. The reason Pacquiao does it so much is probably because he moves alot and feints and lunges so much and uses more footwork than other fighters that it comes up quicker and more often for him.

When the protectors come up, they push them down... Also you can take a good breath ... Maidana does it alot too...

Its the protectors, they tighten up and move all the way up your butt. He probably lowers them to be more comfrable

That or he's just tired haha

frequently putting his hand on his waists and always fixing his shorts in between fights? (i think sergio martinez also does this a lot)

i think his waist has a lot of nerve endings that the garter of his shorts always irritates him it's just my guess tho