A question for the linguists

It feels to me like every time I read about the evolution of a language over time, the general pattern is one of it becoming grammatically simpler. They go from having lots of cases to fewer or none at all, shed moods or aspects or dual forms, even (on the phonological rather than grammatical end) give up on more difficult to pronounce sounds in favor of easier ones.

Which leaves me wondering: when and how do the complicated features develop in the first place? Are there particular conditions (e.g. isolation) under which a language is likely to make itself into a more elaborate system?

Or is this just sample bias, and the pattern I think I've been seeing isn't really a pattern at all?