せい 'reason / cause / fault' in せいなのか 'perhaps it's because of X' hedges the causal explanation. Contrast ~のに 'even though' which sets up the unexpected reality (一見怖そうなのに 'despite looking scary'). ~そう 'seems' after the stem of 怖い marks a visual inference. Note also the distinction between 生やす (transitive, 'grow (facial hair) deliberately') and 生える (intransitive, 'grow naturally / sprout') — the transitive here frames the beard as a deliberate choice.