Hello,
today all of our VMs shut down and rebooted some minutes later.
We are using ESXi 5.0.0 and 3 of 3 Hosts were affected.
All of them had the following Event in the System log: (Translated from german to english.)
"The process C:\Program Files\VMware\VMware Tools\vmtoolsd.exe ("SERVERNAME") has initiated the shutdown for user NT-AUTORITÄT\SYSTEM due to the following reson: Shutdown by Legacy-API."
SOURCE: USER32 - EVENT-ID: 1074
I already googled for the issue but couldn't find help.
Shutdown by Legacy-API could be caused by many different things, but as the Event says, it was initiated by vmtoolsd.exe.
Maybe someone here on the forums can help me. 
greetings,
netstormxls