There are many things that could be given as custom Vim. However, I think this may be your "textwidth" setting.
From Vim Help:
'textwidth' can be set to the maximum width for the line. When a line becomes too long when adding characters, a line break is automatically interrupted.
So, check what the "text width" is set for, and either increase it or set it to 0 to disable this feature.
If this does not work, try what everyone else said, just commenting on all your settings, checking it, fixing the problem. Then slowly turn on the settings again. Maybe use a binary search :)
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