[jmalicki]

I doubt it explains any reasonable fraction of this, but github moving from early adopter techies to general population "normies" would be a reason for the shift. I would expect it explains at least some increase in the use of em-dashes.

[mikkupikku]

Do general population normies really use em-dash, or do they just reach for the dash they see clearly printed on their keyboard?

[zadikian]

I think they're pressing the default dash (actually a hyphen) twice, and that autocompletes to a single em dash.