The theme was words that require all eight fingers (not counting the thumbs as fingers) to be touch-typed on a standard qwerty keyboard. Not exactly a category of the words per se. They all contained the letter p because the letter p is the only one that uses the little finger of the right hand.

Marilyn Vos Savant will occasionally have a theme of this sort, e.g., words that use only the fingers of the left hand.