Hello,
If we check the reason from the log- Diagnostic=
DetectDown - it means the local end enters the Down state when the detection expires because it does not receive BFD packets from the remote end.
Since you enable echo-arm function, it looks like the packets go out of the local end, but it doesn't return.
Since all session go down in the same time, I suspect a physical interruption along the path that converges all 3 session, maybe on the uplink.
So when it happens again, try to ping the remote end of the line, or keep a continuous ping and see whether there are packet losses over that path.