I found where the problem comes from.
Actually, as precised in UIScrollView reference documentation
" The scroll view itself does no drawing except for displaying
vertical and horizontal scroll indicators. The scroll view must know
the size of the content view so it knows when to stop scrolling; by
default, it “bounces” back when scrolling exceeds the bounds of the
The object that manages the drawing of content displayed in a scroll
view should tile the content’s subviews so that no view exceeds the
size of the screen. As users scroll in the scroll view, this object
should add and remove subviews as necessary."
So, I override scrollViewDidScroll in order to refresh map view.