Her motive: greed. And possibly a desire to be as celebrated as Anne Hathaway's Catwoman in The Dark Knight Rises.
Lohan and her assistant (henchman) Sam Doyle were at the house of Sam Magid when Magid called the police to report a jewelry theft. Since then, he has recanted his story, writing a letter to the LAPD saying nothing was stolen.
LAPD isn't buying it: they have witnesses of their own who allegedly saw Lohan and Doyle doing the deed. Lohan is already on probation for jewelry theft, so this latest case could send her to prison. (It won't — she always gets away.)