It was because he was amongst the group of the angels who...

It was because he was amongst the group of the angels who had been ordered to bow down to Adam, and since he formed one of the members of the group, the order also pertained to him as the third 'aya clearly shows that he was 'ordered'. If in a group there are, let us say, more men than women (as in 'Aya of Tat-hir), then the whole group is referred to in masculine gender and the women are automatically included in it, even if the pronouns etc. are masculine.

This rule is called the Rule of "Taghlib" (Rule of Predominance). Likewise, in that group of angels…