Diagnosis
The paper guides in the automatic document feeder (ADF) tray are misaligned or have debris in the paper path, skewing the page as it feeds.
How to Fix It
Clean the ADF rollers. Open the ADF cover (lift the top scan bed). You'll see a set of white rollers. Wipe them with a damp cloth, then rotate them manually to clean all sides. Also, check the small plastic separation pad at the entry and clean any dust off it.
If That Didn't Work
The paper width guides in the AF tray might be too tight or too loose. Adjust them so they just touch the sides of your document without pinching or leaving a large gap.
Heads Up
Scanning heavily textured, curled, or thin paper (like receipts) through the ADF will almost always skew. For those, use the flatbed glass.