Default [Q] Windows Phone Dual Boot (Hypothetical Question)

Okay, I know the odds of this happening are pretty low and at this point, pretty undesirable due to Windows Phone 7/7.8 being so out of date....but, HTC HD2 has been known to Dual-boot with Android and Windows Phone OS (and now devs even have it running Windows RT!), but what is so special about that phone that allows it to run these two OS's but not other HTC devices?