This has come up every other year since PPRuNe began.
Click "Search" at the top of this page, then type in 737+taxi+crab and you will find half a dozen threads on this subject.
If you cant be bothered, I can tell you it is due to the torque reaction of the over-powerful anti-collision lights fitted to this type. They considered a mod which mounted the upper & lower anti-colls so that they rotated in the opposite direction but binned it due to the cost so we live with the crab.