At least three things are going on here:

1. Southwest/Competitor Sales -- Southwest usually starts sales on Tuesdays; sometimes on Fridays. Competitors may have sales at other times which Southwest may match.

2. Changes in Inventory -- Southwest does re-optimize its inventory, usually over the weekend but can happen during the week, too. If they open up more inventory in lower fare classes, the fare essentially goes down. If the close inventory in lower fare classes, the fare essentially goes up.

3. Fare Chanages -- Southwest and/or other carriers may decide to raise fares. Sometimes those price increases stick; other times they don't (which then appears to be an increase followed by a decrease).