Again, it's very sound politics, but that's all it is: political gamesmanship. He's always been a democrat-leaning-republican, which is why A) He's never been that popular within his party, and B) He's had such long tenure in a primarily democratic state. The republicans decided they were better off without him, and decided to see if they could get someone new elected. Specter knew that switching to the democratic party would make him a temporary golden boy to the democrats, and help his chances in Penn. It's a win-win-win for Arlen, the Democrats, and the Republicans, at least until someone loses the general election, when it will become win-win-lose or lose-lose-win. (You can be sure the republican outrage is feigned, most of them probably saw this coming 20 years ago)
It's a simple act of political self-preservation, neither an abominable outrage as the republicans would like you to see it, or a heroic stand against injustice and the forces of Skeletor, as the democrats would like you to see it. Don't buy either company line.