One reason for the "switch" (and I don't guarantee this is the case in Pennsylvania) is that it's so easy to "switch parties" and then switch back again. All you do to "register" is check a box at the polling place to indicate which primary you want to vote in. In some states, anyone can vote in any primary without even checking the box.

In the general election, you vote for whomever you want.

Many Democrats are registering as Republicans simply to vote in the Republican primaries for the candidate (now often Trump) they think would be the easiest to beat.

A Democratic friend of mine voted (as a Republican) for Kasich because there is no way that any Democrat will carry this "red" state in November, he predicted (correctly) that Hillary would win the Democratic primary, and he wanted to cast a "protest vote" for the sanest Republican running.

There's no telling how many Democrats voting for Trump in primaries will support him in the general election.

(Republicans are undoubtedly voting for Sanders on the same principle: they think he'd be easier to beat.)

Trump on Sanders: "He's a Socialist - which is really a Communist, when you think about it."

Get the picture?