To your point with regards to multiple failures combined causing an outage, here's some basic reading on the Swiss cheese model: https://en.wikipedia.org/wiki/Swiss_cheese_model

From over here it looks like the legacy filter was a latent failure, and the BGP automation from the downstream peer of Telia was an active failure (combined caused the outage). Now from the downstream peer's point of view, perhaps the cause of their BGP automation failure was latent also, but we wouldn't know without more details.

Pretty interesting topic.