Didn't know about "Background tasks", thanks
Currently I have only "HTC Hub" there. I will go and check this setting next time when I occur this problem.
But anyway, shouldn't reboot kill all background apps?
Reboot surely will kill all background apps. But these background apps are different to other OS. They are those that will 'start on boot'. So after reboot just in few seconds, they will get to their business as usual.
I love this 'background' task feature! No other OS does it so efficiently as Windows.
The good thing is if you check that place often, you will know what apps are 'going to run'. It doesn't mean they are running all the time. It tends to indicate the capability of them to run.
So as you said you just have HTC Hub, which is same on mine. However, when you to go 'advanced' option on that same page, you will see a list of greyed out apps that are going to run in background and you can't kill them. Reason being these apps need to be told about not running in background by entering them individually.
See if that greyed out list has some huge apps. I downloaded this gMaps thing from market and although I just looked for maps once (a day ago) since then, it's been running. It's because it kept on looking for my GPS co-ordinates. Now that dint freeze my phone (1.5 Ghz, I know!) but it drained battery quicker. Changing attributes within the app, removed it from greyed list and happy days!
In a nutshell, very intelligent task management. You can choose what can run and can't run while your screen is off!

+ you get benefit of battery saver option if you use that so you can choose who can connect to 3g network and who can't while you are not looking at your screen!
PS: whole point of that long post was : Most likely it's not a background task that's hogging your processor, but it's worth looking under advanced and you might know what app was it.