Or maybe he just decided to participate again… 
Stranger things have happened (but I doubt that is the case here) 
As for why... hmm... sold accounts? Or perhaps like SheldonCobb above, having a bitter experience with one or two projects and decided to spare no mercy on the entire crypto platform.
If the account has been traded, it will probably eventually become apparent over the days when the account is actually active but it does not seem it has been sold. The pattern of posting is somewhat counterproductive and that was what stood out for me when he returned to the forum to make an unnecessary post nearly 7 years after his last post.There's chance that such users want to grab attention of other users in order to get merits but most probably they're only spammers and they'll continue their spamming habits even after being inactive for many years.
I could be wrong but that does not seem to be the case with this member. It's entirely possible that someone bought the account as well and is clueless about the trust system or forum rules.
There was not a change of email address but that does not mean a sale could not have taken place. It has been known for some time that email addresses and signed messages are included as part of the account sales process but I am not sure if a sale has taken place here.