In the latter senses, applied to writing as well, but originally it referred to the voice used in public speaking -- as shown in its etymology, from the Latin phrase ōre rotundō, "with round(ed) mouth," dating back to at least Horace (and probably before -- he wasn't the most original of thinkers). Either by association with that style of speaking or by reading the Latin another way, it soon came to mean also "with round(ed) phrases," and so long-winded, bombastic, and pompous.
He mansplained at me for an hour -- gave me flashbacks of Uncle Theo being orotund at Thanksgiving.
Administrivia: I won't be able to post the next two Thursdays. If anyone could pick up the slack, I'd appreciate it.