Relays log a bootstrap warning if they can't extend for somebody else's circuit
Say you have a relay that is up and listed as non-slow in the consensus. Due to the current overload (legacy/trac#24902 (moved)), this relay is getting many many circuit requests per second. Due to bug legacy/trac#24767 (moved), we will make a huge number of connection attempts to other relays that are down, because as soon as we get a "connection refused", we will get another circuit request that triggers another connection attempt.
So when your relay restarts, since it's still in the consensus and clients still think it's usable, it will immediately get flooded with circuit requests, causing these connection attempts to resume.
And Tor calls every one of those connection attempts a bootstrapping attempt, even if there are no origin circuits related to that connection attempt.