Why did HF disolve this lightbulb at the surface rather than more submerged points?
Near surface of the liquid the glass is a bit hotter. All glass of the bulb gains comparable amount of heat energy from tungsten wire, but parts above the liquid are cooled only by air, so they are much hotter. Part of this heat is transferred into slightly submerged part, so it is the hottest part that is under liquid, and all reactions are quickened by increase of temperature. 

However, this exact experiment is a bit ambiguous, as glass here is under extreme thermal stress, and shattering near surface of the liquid may be addressed to that.