It looks to me (not being an 68k expert) that only the first word is considered the "opcode": the second word just selects what "D" registers are used for the CAS operation. Normally one would expect the zero bits to be completely ignored in that case, since they don't have any role in the instruction.

But maybe on the 68030 in this case, the bits must be zero even if they have no documented use, because there is hardwired logic for another instruction that is activated by those bits being set, somewhat like the 6502 illegal opcodes?