Two seperate questions: Rhoticity/Topic-Comment

These questions I have are things I've always been curious about. Does
anyone know why some languages have the tendency to have weak rhoticity or
to drop so-called "r" sounds? The biggest example I know of for this
happening are of course many english dialects that drop "r" and I think I've
read of certain german ones doing it, and also some romance
languages/dialects doing it. But I can never find any good sources
explaining reasons why this "phenomenon" may happen. Any information would
be much appreciated.

Also, is it possible or common for highly synthetic languages to follow
topic-comment patterns and does anyone know of any languages where this is
the case?

Thanks so much,
Adam

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