Well the DB admin had just removed the index a few minutes before, and immediately the API times spiked. When you remove an index there is exactly one risk --slowdown-- and that's precisely what the admin noticed.
"Hey {on-call-engineer}, we're seeing a huge slowdown all of the sudden."
"Was there any change in the last few minutes?"
"We just deleted a database index."
"Yup, that'll do it. Gotta restore it."