There is no preemption support in Cisco FWSM (Or ASA/PIX for that matter) in Active/Passive mode. The primary most probably took over because something won't down on the 'Secondary (Active)' unit e.g. a monitored interface, triggering the failover. Check the output of the 'show failover history' command to see what exactly caused the failover.
In multiple context mode (Active/Active) you can configure pre-emption on a per context basis.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...