why was will taken to the upside down

Will Byers was taken to the Upside Down in the Stranger Things universe after being separated from the normal world during the first-season events. The exact mechanism is fictional, but the core idea is that his disappearance coincided with the barrier between dimensions weakening or being breached, allowing him to be pulled into the alternate realm where the Upside Down exists. As a result, the Upside Down became a mirror-inverted, dangerous version of Hawkins, and Will’s absence from the real world became the catalyst that linked him to that dimension for an extended period.