A) He held the bat way, way down low, with his lower hand well past the knob.
B) The bat looked incredibly long, although the length may have been accentuated by how low he held it
C) His swing seemed remarkably unlevel for such a successful hitter.
Click through for some baseball card images I grabbed confirming how low he held the bat.
1981 Donruss #94
1981 Donruss #214
(while we're at it, why did Yaz have two '81 Donruss cards?)
1980 Topps #720
1981 Fleer #638
1982 Fleer #312
You can actually see the knob on this one
1992 Score The Franchise #3
This one he's actually much more choked up, but he's also very young in this photo. Perhaps he adjusted his approach later on.
1993 Ted Williams #7
Great shot here of a totally "disappeared" knob