Why does it render in the background when I drop a clip into the timeline and have already selected

I must be the last person on Earth to convert from FCP7 to FCPX. Anyway I have a question.

In FCP7 I never had to render anything if the sequences settings matched the clip settings. But now when I drop a clip into the timeline and have previously checked the Project settings to: Use Automatic Settings "Video: Set based on first clip properties" When I drop it into the timeline it starts background rendering. Why?

First, switch off background rendering.

Maybe you're using a non-standard clip format that FCP doesn't recognize. What are the clip specs and the project properties?

Yes I was using a non-standard clip format that FCPX didn't recognise. When I'd drop it into the timeline it says that it's not recognised and I was pressing OK and it would render. However I figured that if I clip the drop down menu and click custom it puts in the right sizes for me so now it doesn't render.