Linux kernel remounts the filesystem into read-only mode whenever it cannot process I/O. This might happen due to various reasons such as disk failure, SAN connectivity issue, disk with bad blocks etc. On Virtual machine and SAN based storage environment even the high latency may lead to I/O hung and result in read-only mode.
Debugging pacemaker cluster through logs will be much more easier post reading this article. This would help you in either troubleshooting or root cause analysis. It does not need any additional tools except your attention. Have reproduced and collected logs from various scenario to present here as example. I used SUSE 11 with HAE system for testing.
The first thing to do is look in system log for the terms ERROR and WARN.
#grep -e ERROR -e WARN /var/log/messages