If VMware detects issues it will display the purple screen of diagnostic (PSOD) rather then reboot. So you may need to check your hardware. Vendors like HP have ASR (Automatic Server Reboot) enabled in the BIOS which tracks the system health and initiates a (hard) reboot in case it detects an error which might be resolvable by rebooting the host. However, such issues are then documented in the hosts system management (ILO in case of HP). Other vendors have similar features built in.
André