Slave claims requested WAL segment already removed - but it wasn't

Hi-
We have a master (pg 9.4.4 on Ubuntu 14.10) and a slave (pg 9.4.8 on
Centos 7). During a period of heavy use, the slave began complaining
that the “requested WAL segment xxxxxx has already been removed”. But
the WAL segment was still on the master. The issue was resolved by
manually copying the pg_xlog directory over to the slave.

I don’t see any errors on the master log file, or any other messages on
the slave’s. What happened? How can this be prevented in the future?

Thanks,
Jeff