Weird Mouse+Alt moving in editor

[FlawlessHappiness]

I wanted to study how the map-structure in 22_torture_chancel, but a problem happens to me:

When i look down on the map i am able to (While holding left mouse button and Alt) slide across the map left, right, forward and backwards, as usual.
But when i look from the side I can go left and right, but also up and down. This irritates me a lot, because it makes it much harder to move across the map. Why is this happening with some of FG's maps? Can i turn it off?