The 65C265, unlike the 65816 which it's based on, has a pretty bizarre boot sequence where you need to put the string "WDC" at 0x8000 if you want to keep the processor from booting into its built-in monitor ROM.
That's actually the onboard monitor ROM doing that. It will hand off to your ROM if it sees that signature. It's analogous to how, say, the Commodore 64 hunted for cartridges (if it saw "CBM80" at $8004, it used $8000 as a vector and ran the cartridge ROM instead of entering BASIC).