How do you explain PDA?

I was in the grocery store looking at the selection of pre-packaged salads. This couple I had never seen before pushed their cart in front of me so that they were in between me and the produce I was looking at. Then, instead of picking out salad as I would have expected, they started kissing very noisily (a lot of smacking and moaning). What was that about? I don't get public displays of affection. Someone who does that please explain.