My guess is the stain wasn't really dry/cured. DAMHIK. You didn't say what the temp. and RH were where you are working. If the temp. is less than 60F and the RH greater than 60% it can take a looooong time for OB stains to cure enough that you can topcoat them using a brush.

Strip off what you've already done and start over. Don't sand it, you've sanded it enough. Get some KleanStrip Premium and follow the directions. It'll be off there in 30 minutes. Neutralize, let dry, then start anew. Make up a test board at the same time. Use the test board to prove when the stain has dried enough to be top coated. I'd wait at least 2 days before even thinking about it.

John