Sync needs additional 19.3 GB

This morning my iPad synchronized with no problem. I haven't changed anything since then.

This evening sync fails, needing additional 19.30 GB. I had to uncheck music, photos, & videos to get it to sync. Then I checked them again, and got the same graphic showing 2.03 GB free, and it synchronized.

Back to where it was this morning.

"other" is only 1.89 GB. It's taking a while to synchronize my audio, back after deleting it.

How do I trace down this issue and fix it?