I found a few reasons why Ph changes:
1. Temperature. It's not measurable with most tests. 17 degrees drop just to knock off .04 from the PH. Just thought i"d mention it.
2. Rocks. Water causes rocks to disolve and it's thought they leech lime into the water which decreases Ph. If you have corals, dolamite, limestone
3. Tap water. A recent water change can increase or decrease Ph based on the Ph of the water being used.
4. Over treatment of Ph up or Ph down.
7.4 for is a good Ph for a lot of tropical fish. You water may settle to that level based on everything in the tank. A .4 change near neutral, 7.0 is not the dangerous a change in 24 hours.