Streaming replication problem

I'm using PostgreSQL 9.2.4 on x86_64-unknown-linux-gnu, compiled by gcc (GCC) 4.1.2 20070115 (SUSE Linux), 64-bit.

I set up streaming replication for a read/write primary and a read/only
standby. The replication works fine for a while, and then out of the
blue BOTH machines become read/write, but with no replication from the
original primary to the newly read/write standby.

The only log entry that seems relevant is as follows:
FATAL,57P01,"terminating walreceiver process due to administrator
command",,,,,,,,"ProcessWalRcvInterrupts, walreceiver.c:150",""

Any help/guidance would be appreciated. Thanks in advance.