Perhaps he got too close with the Brock Pierce investigation!
Could be... he started poking into the dirty doings of some big shots like Pierce, then of some really big shots (of the kind that employs a dozen lawyer full-time), then of some really, really big shots (of the kind that appoints Supreme Court judges) ... He may well have received a letter from a lawyer firm with a dozen names in its name. He does not seem to be rich enough to afford a legal battle over that.
But, just before he left, he also had painted himself into an awkward corner over an old claim that he once lost over 1000 BTC in the InstaWallet "hack". He had been demanding refund from the InstaWallet ex-owner, but the ex-owner claims that there is no record of those BTC deposits in their database or the blockchain. (But, on the other hand, it seems quite possible that their search procedure was inadequate.) The InstaWallet ex-owner is an admin of this forum, by the way. Phinneas offered a bounty for the return of even a small part of his BTC, but it was phrased in such a way that the thief could just give him that small part, minus the bounty, and walk away free with the rest:
https://bitcointalksearch.org/topic/new-400-btc-bounty-pales-roger-vers-376-btc-bounty-for-return-of-stolen-btc-649176.
But also someone claimed that Phinneas owed him a refund for a used mining machine that he sold through an Internet auction and was damaged on arrival:
https://bitcointalksearch.org/topic/phinnaeus-gage-aka-bruno-kucinskas-debt-paid-7192014-667460.
And his last log-in is exactly three years and 1 day after his first one. Perhaps he realized that, and thought that it was a good time to quit the habit.