Why do radiators come on when I run hot water?

A make and model would possibly be helpful but it sounds as if the diverter valve(if in fact your boiler has one)(do you see my point about make /model)has stuck partly open or is damaged. EDIT this does sound like the diverter valve is partly stuck open and is allowing water to p to the radiators when running the hot water> I have just noticed after running a bath thar my rads got warm as well and I have a Worcester boiler. They use the same diverter valve. Legaly speaking its a job you could do yourself as it doesnt involve any gas or flue connections but unless you are a really good handyman I would advise you get a heating engineer in to have a look. It may be possible to strip the valve and clean it out but it depends on its condition or it may just need a new diphragm and a little silicon grease to solve problem. Its one of those need to be there to see exactly whats happening and fix accordingly. I will have to have a look at mine asap and will definately know whats going on then.