If you read the article it's quite clear why there were concerns and coercion doesn't have to include physical threats but I would have thought most would already be aware of that:
One issue in the case was whether the coach’s decision to pray in such a prominent place, on the 50-yard line, amounted to a private moment of giving thanks or a public demonstration of his religious faith that his players may have felt compelled to join.
Kennedy urged the Supreme Court to find that he was acting on his own behalf, expressing his own religious views, not speaking as a mouthpiece for the school. But the school district said the students on the football team looked up to their coach and felt coerced into doing as he did.
"Today’s decision is particularly misguided because it elevates the religious rights of a school official, who voluntarily accepted public employment and the limits that public employment entails, over those of his students, who are required to attend school and who this Court has long recognized are particularly vulnerable and deserving of protection," Sotomayor wrote. "In doing so, the Court sets us further down a perilous path in forcing States to entangle themselves with religion, with all of our rights hanging in the balance."