alt + r, g doesn't reset. after in and out in edit mode same weird rotation problem occurs.
You have keyframes on your bones. You can examine them in the action editor, which you have opened on the left side.
After resetting the transformations, you are entering edit mode, then entering pose mode. On reentering pose mode, the fcurves are reevaluated. Since the keyframes are extrapolated horizontally your pose bones have values.
Remove the keyframes to completely reset the transformation. Or reset the transformations and create new keyframes with I.