Sounds like a -10 from position 3. Which in turn sounds like you got a manual review once you broke the top 3, and they found something problematic.
I would guess its something minor, like a marginal case of keyword stuffing. So, not completely unnatural, but found in H1, alt tags and everywhere else. Or hammering the same anchor on internal links. Something on-page and easily fixed.
Evaluate the page/site, decide what can be improved or fixed, and do it all in one go. Then sit back and wait at least a month, and probably more like three. Possibly even more if it is your homepage.
If it IS your homepage, you might want to take a close look at how you link back to it.