I'd say something is making the sliders works.
If you press on your keyboard Shift + E/W (to increase decrease tempo on Deck1) or shift+I/U /same for Deck2) do they move accordingly and stop? (don't connect your SB2 for this to test)
What if it works like this I'd say your ddj-sb2 is faulty as it is "short-circuit" that slider or something like that.
My ddj-sb2 is working great. If you need me to test something so I can try to reproduce the problem just say so. But somethings are better watching so a video of what's happening can help. Just make it unlisted when you upload to youtube and post the link here if you feel comfortable with this.