Software is written with a context, and the context degrades. It must be renewed. It rots, sorry.
When you look from the program's perspective, the context changes and becomes unrecognizable, IOW, it rots.
When you look from the context's perspective, the program changes by not evolving and keeping up with the context, IOW, it rots.
Maybe we anthropomorphize both and say "they grow apart". :)
We say the context is not backwards compatible.