Early in Season 5, Pastor Jeff decided that the church needed a youth pastor. Pastor Rob was a great fit in the church, and because he worked with Mary, they spent a lot of time with each other. It was innocent enough at first, but it wasn't long before Mary started to have feelings for him. While she did her best to suppress those feelings, they became more evident in "Future Worf and the Margarita of the South Pacific.
After being kicked out of the church, Mary needed a new job -- so she applied at the bowling alley where Brenda worked. That was already awkward for Brenda, but it became awkward for Mary when Pastor Rob walked in with a date on his arm. It likely came as a shock to Brenda after she noticed that Mary was being "sweet on him," since she was supposed to be happily married. It's certainly possible for Mary and Rob to have an affair, which in turn may explain why Brenda puts a move on George.
Brenda told Mary that she regularly saw Pastor Rob bringing different women out on dates. While that might be true, it might also be part of Brenda's plan to get with George. She could see that Mary was eyeing Pastor Rob and saying something about Rob having multiple dates would make Mary jealous.
Brenda could tell George about Mary's feelings for Rob and start a fight. Between Mary's jealousy and George's outrage, he might do something irrational -- like having an affair. Brenda has always been quite the pot-stirrer, and knowing that there's a weak spot in the Coopers' marriage gives her an opportunity.
