On possible explanation is that in this particular case the owner had resided in the property for six years after the foreclosure, during which time the property was conveyed from one lender to another without, presumably, any effort to remove the former owner. Then the most recent owner attempted to have the former owner removed by legal process. The court referred to the owner as a ‘tenant at will’, perhaps conceiving that since no owner had attempted to have him removed for six years his occupation ripened into some kind of consensual tenancy.
Of course this is at odds with the normal concept of tenancy at sufferance, and likely any conception of a tenancy at will, which would appear to require some manifestation of assent to the continued occupancy beyond mere forbearance from eviction. This does not appear to be a legal justification for the decision but a practical explanation.
Perhaps the judge would have ruled differently had the eviction been initiated within thirty days of the foreclosure? Or if the foreclosing lender had immediately sent the former owner a notice advising him any continued occupation was non-consensual?