One of mine does the same thing and I find that she does it most when the nitrite levels are beginning to go up. If your levels are fine then your water may just require more oxygen or your axolotl could simply just like hanging out at the top. If it makes you feel better, my two spend about 60% of their time at the top as opposed to on the bottom and I think they're sleeping half of the time because they don't move for hours
