Implementation-defined behaviour

Ran into this excellent post by Eric Lippert on MSDN about implementation defined behavriour. Excerpt:

An example of undefined behaviour in C, C++ and C# is writing to a dereferenced pointer that you have no business writing to. Doing so might cause a fault that crashes the process. But the memory could be valid, and it could be an important data structure of the runtime software. It could be code that you are now overwriting with code that does something else. And hence, anything can happen; you’re rewriting the software that is running into software that does something else.

By contrast, an idiom that has implementation-defined behaviour is behaviour where the compiler author has several choices about how to implement the feature, and must choose one. As the name implies, implementation-defined behaviour is at least defined. For example, C# permits an implementation to throw an exception or produce a value when an integer division overflows, but the implementation must pick one. It cannot erase your hard disk.

All that said, for the rest of this article I’m not going to make a strong distinction between these two flavours of underspecified behaviour. The question I’m actually interested in addressing today is:

What are some of the factors that lead a language design committee to leave certain language idioms as undefined or implementation-defined behaviours?