A rule becomes difficult to question when obedience is treated as evidence of virtue.

Once compliance is treated as a sign of good character, questioning becomes a character flaw rather than an argument. The rule stops needing to justify itself, because anyone who asks it to has already been reclassified as the problem. Whatever the rule was originally for is now protected by something much stronger than reason, which is the fear of looking disloyal.

Watch for places where a question is answered with a comment about the questioner. That substitution is the signal. And if you hold any authority, make it visibly safe to ask why, because a rule that cannot be questioned will outlive its usefulness by years.