Sh!t happens, and multiple drive failures are not unheard of, especially if the first failure was not addressed on time. SMART tests are better than nothing, but are not guaranteed to catch all possible issues.
The good point here is that the pool is “only” degraded. Raidz2? Raidz3? Stripe of mirrors?
The backup drive may have developped issues due to unreliable USB connection.
You should definitively investigate the condition of the removed drives (still on? SMART report?), consider backing up the pool anew, and check the backup. In no particular order, as I do not know what is most critical.