What do the following words have in common?

HINT #1: A hint to the solution and this puzzle's title: CONFUSED

DO
ID
PI
COO
DUE
HOW
SEA

HINT #2: Think past tense.
HINT #3: The same three letters can be added to each word to get new words.

Answer:

The letters/word LED can be added at the end of each word to make a new one:

DOLED
IDLED
PILED
COOLED
DUELED
HOWLED
SEALED

HINT and puzzle title: ADDLED (ADD LED) 

Credit:
Lee Glickstein and Nancy Salomon (adapted)