to start-- there is no official answer, because we don't actually know how middle chinese sounded. we have educated guesses, but its always good to keep in mind that at least some of the world's theories are probably wrong. middle chinese has never been heard even by the oldest living people's great great great grandparents.
行 has five recognized pronunciations in standard mandarin-- háng; hàng; héng; xíng; xìng. I did not find any prevaling theory on the specfics, but we can talk about why these kind of changes happen in general.
originally it meant crossroads, which slowly changed over time to mean roads itself, and then walking/travelling itself. it can also have a variety of other meanings- the meanings themselves aren't relavant but the fact they exist may be.
language is not a clean and tidy thing. sometimes there are very logical changes to a words use, and sometimes it is not, it just changes organically. some examples of what it could have been:
a separate word was merged together and combined meaning.
the word it used to be was often used for something else, and so there was a natural change in vocabulary.
some other language influenced chinese at that time and was brought into the language.
and many many more. none of these are chinese specific, but things that happen every day in languages around the world. they also happened in the past too.
please note, it is also possible that it was always there to begin with and we don't have a good record/theory for it. the definitions associated with xing have been documented since at least early middle chinese period, if not older than middle chinese itself.
its far less conclusive, but also note that both cantonese and mandarin have two completely different pronunciations of this character, depending on definitions. so regardless of the reason that happened, the chance of it existing in a shared past language (middle chinese or older) is very plausible.