why there is a large file named core in wpmu’s directory?

Files named “core” are created when some OS process crashes.
In your case it was likely a crash of the php interpreter.

These files being memory dumps are used to debug the process “postmortem” – e.g. to check in which function did it fail, so it is perfectly safe to delete them if you are not going to debug the issue.

In fact, on many systems there is a setting to not allow creation of core files by default.

BTW, the date of the file indicates the date of the original process crash.

More info: core files

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