After screenshotting using any of the following combinations:

• Command + Shift + 4
• Command + Shift + 5
  the window resizes when you move your mouse despite not holding down the resize shortcut (I have it set to Command + Shift). To stop the behaviour, you must press Command + Shift again.

To replicate this, set your resize shortcut to Command + Shift.

I'm sure it is not intentional behaviour, and no one has probably used this shortcut before.