Tug-of-War!
Why must a robin tug so hard to get a worm out of the ground? A worm's anatomy is the clue to the answer:

A worm's body has hair-like bristles called setae that help the worm crawl and also to grip and anchor itself firmly in the ground. Worms can grip and anchor so firmly that robins must tug to get them out of the soil.

Image: Wayne Kryduba