Slave I/O thread killed while reading event (no replies)

Hi,
I have 3 slaves connected to my master database. The slaves had been running ok, but then replication stopped a few days ago, so I restarted the replication. Today they've stopped again, with this appearing in my MySQL logs on the slaves:

130309 2:01:01 [Note] Slave I/O thread killed while reading event
130309 2:01:01 [Note] Slave I/O thread exiting, read up to log 'mysql-bin.000758', position 166668826
130309 2:01:01 [Note] Error reading relay log event: slave SQL thread was killed

That's all I see related to the error - I don't see any errors related to a specific query. There are no errors in the master's log. Any ideas how I can diagnose the problem?

thanks,
imran