I came in this morning to find my primary file and application server
(NetWare 6.5.6) was unavailable. The DATA volume had been dismounted and the
pool it resided in had been deactivated. There were a number of errors on
the console screen and logger screen, so I restarted the server.

These are the two errors that came up during restart.

NSS-3.00-5002: Pool MAIN/POOL1 is being deactivated.
An error (20012(nameTree.c[45])) at block 6753664(file
block -6753664)(ZID 6) has compromised volume integrity.

NSS-3.00-5000: Volume MAIN/DATA is being deactivated.
An error (20012(nameTree.c[45])) at block 6753664(file
block -6753664)(ZID 6) has compromised volume integrity.

I ran a successful pool rebuild, reactivated the pool and remounted the
volume. Where would I look to find what may have caused this problem?

Is it likely to happen again if I don't find and fix the cause, or is it
just something that happens and needs to be dealt with?

My server had been up for 370 days before this happened.