This behavior is known as “polar mode”. Polar Mode is similar to Polar Tracking in AutoCAD. You can turn polar mode on or off by clicking the button in the measurement toolbar.
When adding points in polar mode, the point is forced to fall on a line that is a multiple of 90 degrees from the previous point. This is most commonly used to create horizontal or vertical lines. It can also work for lines of various angles. To adjust Polar Mode Settings, select the Settings Tab and User Preferences then click the Drawing Tab. You can disable polar mode when creating measurements when Counting and you can choose snap angles.