I only watched the last episode so I don't know who any of the people were.
Couple 1: They both recite their vows, and when asked to say "I do" the girl backs out at the altar. The guy seems to take it pretty well.
Couple 2: They both recite their vows, and when asked to say "I do" after the girl already did, the guy backs out at the altar. The girl is completely crushed.
I watched no other episodes before tonight, and my take is that the two people who reneged at the last second just were part of the popular desire to be "on TV", with no serious intent to go through with an actual wedding.