I was making a wall hanging and the fabric bunched up on the back. NowIhave to take it apart. How can i avoid that?

lots of basting is only thing found that helps my bunching issues.
Im still a beginner, so not much help, but there are ALOT here that can :D

Did you have your feed dogs down?

If you are pinning start in the middle and work your way out.I find this is the best way.

Use a temporary adhesive spray to baste.And like Darlene, said.

On a small project I use blue painters tape and tape the backing wrong side down on my counter or table. Next I smooth out the batting on top of that then put my top, right side up and tape it. I tape it smooth without stretching it out of shape then use quilters safety pens and pen it all over starting from the middle and work out, I pen it every 3 inches or so.

Welcome aboard. I hope you love the quiltingboard as much as I do.
I support all of the above posts.
Are you using a different weight of material for your backing?
If it is a light weight backing and you have the height and weight of the batting and topside fabric between it and the pressure foot, the feeddogs may be working faster or slower than the needle. Veterans, did I get that right?
I think a quilting foot stabilizes that situation but I have never used one so I don't know for sure.
Can you make a sandwich from scraps to do test runs on?
Make a run with just top and bottom fabric. Bunching?
Add batting and make a run. Bunching?
Drop the feed dogs and make a run. Bunching?
Adhese or pin at 3 or 4 inch intervals and make a run.

If it is still bunching:
Flip the lining to the topside and repeat the test strips above. Does it still bunch? On the needle side or the feeddog side and on which test run did it start to bunch?
If it is still bunching after all the test strip you may have to adjust your tension for this one project. I need the veterans to coach you on that so please let us know if it gets down to that.

Heavily starch the backing and then baste it VERY well with spray, pins, thread, your choice, but close together with the pins/thread. I even pin around the edges if I spray baste.

tons of basting.

tons of pins, starting from the middle out. slow and steady. quilting foot definitely helps!