This was all caused *solely* by not releasing water quickly enough.
Reasons for not doing that:
To protect the main spillway, which was known to already be weakened/damaged.
To protect the downstream levees from increased flow.
To keep a high level in the reservoir (yeah, pretty stupid but that’s the viewpoint they’ve had for a long time).
Also, this may have been an intentional effort to briefly test the emergency spillway in case it had to be used.
And, protect the little fishies.