I think we have a problem, Houston.
I happened to be watching the 'top' of running processes on my VPS and noticed optpng ran when a huge bunch of PHP processes were being spawned, to the point of throwing the website server and mysql offline.
I'm only presuming here but think it's the scheduled optimisation process for constantly optimising the theme folder, buddypress and the two additional folders I have configured.
I've disabled this now and will monitor.
 Any reason to optimise the theme every hour?
 Same for Buddypress
 Any way to trigger the additional folders manually (when I want)?
While I'd like to trigger additional image folders manually, I think it's far better to have seperate (or definable) time periods to optimise. Every hour is a little hardcore (for me anyway)