He forked, stripped attribution and history [+] (license violation, but on its own probably just rises to the level of minor dickishness and is often a misunderstanding)
He started advertising his fork to replace the originally forked repo (a bit rude).
If the original library was unmaintained or unused, this probably would have been a big nothing-burger. But apparently the library that was quietly forked had a fairly big install base and contributing to that library first would have been a better choice.
What is missing from the discussion (and often unaccounted for in licenses themselves) is that there's a massive personal value to having a library used by large numbers of people.
[+] EDIT (to clarify): as gcr points out, this was not a standard github-style fork but rather a copy and re-commit as original history.