Why do I hear crackling in my ear when opening wide?

Pressure change. When you open wide, a muscle called the levator palitini stretches out the tube that connects the middle ear to your throat. That makes sure it opens wide to let air enter the middle ear. Air movement equalizes the vacuum created when your middle ear skin absorbs oxygen from its air. It is a simple benign process. If it doesn't bother you ignore it.
Eustachian tube. Your symptom is related to dysfunction of the eustachian tube that connects the back of the nose to the middle ear. If the problem has persisted for more than 2 weeks, i recommend you see an ear, nose & throat specialist.