That “personally” emphasises some matter is subjective, that you are not treating it as incontestable truth.

I’m not certain but I also think it’s increasing in usage. And I also think it’s part of a trend where people pre-emptively shield their statements against assumptions others might come up with. You know, people really love to screech at the others not because of what the others said, but because of their assumptions = lies = bullshits over what the others said.

I also believe a few other ongoing trends are basically for the same reason: “though” at the end of sentences, “basically” (eh, self-demonstrating example), the modern usage of “literally”, etc.