I honestly do not think this email is telling the whole story. When was the falling out with you and this person and why did it happen? Clearly he is inferring that you know what he's talking about related to something that happened previously. it really doesn't sound like a rant that's coming out of nowhere.

I'd honestly reflect on the recent past and try to figure it out.

No one here can really do that for you because there just isn't enough information here.