Brita "pitcher" filters reduce copper, mercury, zinc, chlorine, cadmium, lead, asbestos, and benzene, as per their User Guide. Most of that stuff should not be in your tap to begin with. I cannot find any claims to lower pH.
If you home has copper pipes you will have copper in your water. My tap water has 0.06ppm of copper in it. Some older homes actually have Lead pipes. Water utilities are required to add stuff to the water to limit corrosion of pipes. A few years ago flint Michigan decided to stop adding corrosion inhibiters to the water to save money. Lead pipes started to corrode and people got sick. Eventually people found out and some people in the water utility convicted . Flint is now replacing lead pipes with Copper. They hope to be finished in a couple of years. Orthophosphate is frequently added to water to inhibit corrosion of pipes.
Chlorine is commonly used to kill bacteria in water. There is a 99% chance chlorine is in the northern california water you drink.
Abestos is a type of rock found in nature. Some Well water is sometimes rich in asbestos and or, arsenic and some heavy metals .
Zinc is a plant nutrient and is typically found in small amount in streams and rivers. Cadmium Mercury can also come from rocks or pollution. Benzene is present in gasoline. Small spills from cars and motor boats can often result in very small amounts in drinking water.
Water utilities do what they can to minimize these contaminates so that the water is safe to drink. But problems can and do occur from time to time and sometimes the contaminate levels go up. Water utilities often test their water to catch and correct problems before you know about them. Some small community water utilities only provide suitable for hygiene, cleaning and gardening and recommend that you do not drink it.
I don't know why his PH dropped so much. We simply don't know enough about the filter and what was in the water to determine that