As Steve and 4thGen pointed out, its about flood control. The water managers have an incredibly complex j ob of balancing competing interests, but whatever the rules of engagement, the bottom line is quite simple. If they are wrong on the side of letting too much water out, they POTENTIALLY contribute to future water shortages if future rains do not materialize, resulting in challenges for residents, farmers and ecosystem health. If they are wrong on the side of not letting enough out and the dam overtops (which came within hours of happening in 1955, when the consequences would have been far less costly), they CAUSE catastrophic flooding resulting in billions of dollars in damage and likely many lost lives. Prudence says you err on the side of letting too much water out.
Yes, they were "dumping" almost 35,000 cfs yesterday, and continue today. But the inflow is over 62,000 CFS. So the net inflow is almost 20,000 cfs. Or nearly 0.5 acre feet per second = 43,200 acre feet per day. At that rate, the lake would fill up before Christmas! I'd be dumping a lot of water, too, if I were managing the dam.