They are infiltrators. Democrat lawyers began registering as republicans in heavily republican districts about 10-15 years ago to try and soften the GOP underbelly in the heartland.
They’re moles. Period.
In this county, the Board of Elections is run almost completely by black Democrats. However, when they learned that teams consisting of the same number of Democrats and Republicans must be present for many tasks, such as taking ballots to nursing home shut-ins and recounting contested elections (for which they can be paid around $300 a day), suddenly there were a whole bunch of changed registrations — to Republican.