This seems like it may be a bug in the update that HTC has released for their phones. It's been reported on the Android issue tracker and appears to affect various HTC models only. You can star the issue if you want, and some people have suggested that enabling the "Fast Boot" option in your settings may help (something like Settings->Power, I believe). On other devices it should only run at first boot or when the Dalvik cache has been erased.
If it's of interest, the details regarding what the optimization is actually doing have been covered in What kind of app optimizations do newer Android versions do at the first reboot?
