When moving a window by left-clicking at the top of the window and dragging up, or to the left or right, the window will re-size as soon as the pointer reaches the screen edge. For right or left the window is placed at the corresponding side of the screen, but for a top of screen "hit", the window will be maximized. Because the pointer is so near the upper border of the window, it is difficult to avoid triggering the full-screen switch when all I want to do is to position the window at the top of my screen.
Its called Aero Snap. I also find it a not so good feature and had the same problem as you. Not to worry.
Control Panel > Ease of Access > click make my mouse easier to use and then put a tick in the box next to prevent windows from being automatically arranged when move to the edge of screen and it will turn Aero Snap off.