The likely reasons why Disney would sue is if someone is making money or gaining notoriety off of a clearly recognizable Disney creation (like the castle), or if something that is recognizable as Disney was presented in a negative light or associated with something salacious or otherwise contrary to Disney's owners' best interests.
One other likely reason is if someone outside of Disney brings a trademark violation to Disney's attention (for any image that includes something that Disney has trademarked). In that case, regardless of the circumstances, Disney has no choice but must take action to protect their trademark. They can use their discretion if they discover the issue internally (as long as they don't keep discoverable records), but if someone external knows that Disney knows that their trademark is being used without permission, Disney has to do something or risk losing their rights.