What would make a guy put his hands on another guy's waist?

Sometimes when my friend from work comes in on his shift and sees me he comes from behind me and places his hand on my waist and says, "What's up buddy?" (or something similar to that). Once I was bending over to get some ice and placed both of his hands on the sides of my waist as he was trying to get by me because I was in the way and I kind of turned my body to the side to move out of his way. If I'm in his way he'll try to squeeze by me but he'll place his hand on my lower back area while doing it. This has happened more than once and I'm not sure why he does that. My mind might just be in the gutter but I still wonder, is it because he's just secure in his uality and he doesn't really think about it when he does it...or is it just a habit? I don't think he's gay, he's dating my friend. What do you think? Is it normal for a guy to do that?