It's reminiscent of ARM, but the relevant part is that the CAS instruction's second word bits 5:0 look like a "modrm" (to use the x86 terminology) where the officially documented values select only Dn, but the undocumented variant would correspond to (d16,An). At least, that's my theory for why A1 gets modified.
It also appears that I may not have been the first one to discover that something odd was going on with that bit, causing it to use A0-A7 (with weird results) instead of D0-D7:
http://goldencrystal.free.fr/M68kOpcodes-v2.3.pdf
It's reminiscent of ARM, but the relevant part is that the CAS instruction's second word bits 5:0 look like a "modrm" (to use the x86 terminology) where the officially documented values select only Dn, but the undocumented variant would correspond to (d16,An). At least, that's my theory for why A1 gets modified.