"--sync" goes to the gtk/gdk toolkit (the libraries that come with the OS). It switches them to run synchronous instead of asynchronous.
(see the issue report I linked earlier => the OS dumped a message in which it points out the "--sync")
"significantly reduce"
Actually, my current guess is, that it is a race condition (gtk/gdk has several threads).
E.g. when I run lazarus normally (without --sync) it does not always happen. Sometimes it only happens on the 2nd or 3rd "no". So it happens with a certain likelihood. (as race conditions do)
Running synchronously may fix the race entirely, but it could also just reduce its likelihood of going wrong.
Mind, that is "my guess" only. I don't have enough background on gtk/gdk.
Well, then of course, each Linux distro, and each version thereof (or rather each version(s) of gtk/gdk) may react different.
On some it may always work flawlessly. On others it may break, but with different likelihood....