If Ulmer is to be believed, they just robbed her. 

So they robbed her when she was en route to her wedding to Rhaegar?

Do you think that’s when the Crown decided things had really gotten out ofhand, and sent Selmy and Dayne to deal with the Kingswood Brotherhood?  

Text doesn’t say, but I would guess it would have been a major embarassment for the Crown so it probably moved the issue up the chain.