Busy armchair engineer here, 100% guaranteed pure speculation on my part
According to this article the exact issue came up 12 years ago when the dam was re-licensed. When the various officials have been asked on TV about this 2005 report it's been like watching the three monkeys, no one admits to having seen it.
Looking at the pictures, and diagrams, and the DWR's panic about erosion it appears to me that the emergency spillway wall doesn't have anything like the kind of foundation that I would expect for a "mini-dam".
They fixed some cracks in that area (apparently with some kind of grout/caulk) a few years ago, and inspected it remotely a couple of years ago. Talking to my neighbor this morning we agreed that neither of us had seen any sign of rebar in the concrete floor of the spillway. With broken concrete like this wouldn't you expect to see the rebar?
Heck yeah, there would be rebar all over the place, especially since the rebar should either be tied or welded together at each junction.
I'm no dam expert, ha, but I've seen a few in my day, and I would expect a wider spillway on such a large lake.
Wonder if the concrete was even tested as it was poured back then, doubtful. Then did someone decide to add more water to the mix as it was being poured, and weakened it. Ugh, Government at work always give me heartburn.