[Tweeters] Black-capped chickadee behavior question

[Ethan Whitney Smith via Tweeters]

I have plenty of guesses about what I witnessed, but no educated ones. I
saw two black-capped chickadees flutter to the ground in my back garden.
One was immature and the other an adult. The adult was pushing on and
pecking at the immature. The immature chickadee lowered its head and
rapidly flapped its wings. The adult pecked at the immature chickadee's
head repeatedly until the immature was dead. The whole thing took maybe
15-20 seconds. Anyone have an educated guess about why this happened?
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