Wh-signs can be used in non-interrogative contexts when they introduce temporal clauses (SYNTAX 3.5.2) locative clauses (SYNTAX 3.5.3), manner clauses (SYNTAX 3.5.4), and reason clauses (SYNTAX 3.5.5).
Notice the sign glossed as why is identical in its manual parameters to the sign glossed as reason, which typically introduces reason clauses. However, the two signs differ in terms of absence/presence of specific non-manuals (SYNTAX 3.5.5).