When you cast a concentration spell, ending the last spell you were concentrating on.
(comicpress.socksandpuppets.com)
Well it has to be a bonus action cleric spell with concentration because throwing an object is an action, so the options are: Shield of Faith, Holy Weapon or Spirit Shroud. She's not holding a weapon, and it doesn't look like she's summoning spirits of the dead, so I'm going to go with she is casting Shield of Faith.