Sometimes the mouse pointer moves erratically. This is because of ___.

Through normal use, a mouse will accumulate dust, lint, and oils on vital operational parts. For a mouse that uses a ball, the internal rollers will become dirty.
For an optical mouse, the optical lens and possibly the LED lens will become dirty. This will cause performance to slowly degrade. If the mouse pointer begins to move erratically or the mouse itself no longer moves smoothly, the mouse probably only needs to be cleaned.
Source: Dell technology