From Matthew Henry's Commentary...

"She ought to have power on her head, because of the angels. Power, that is, a veil, the token, not of her having the power or superiority, but being under the power of her husband, subjected to him, and inferior to the other sex. Rebekah, when she met Isaac, and was delivering herself into his possession, put on her veil, in token of her subjection, Gen. xxiv. 65. Thus would the apostle have the women appear In Christian assemblies, even though they spoke there by inspiration, because of the angels, that is, say some, because of the evil angels. The woman was first in the transgression, being deceived by the devil (1 Tim. ii. 14), which increased her subjection to man, Gen. iii. 16. Now, believe evil angels will be sure to mix in all Christian assemblies, therefore should women wear the token of their shamefacedness and subjection, which in that age and country, was a veil. Others say because of the good angels. Jews and Christians have had an opinion that these ministering spirits are many of them present in their assemblies. Their presence should restrain Christians from all indecencies in the worship of God. Note, We should learn from all to behave in the public assemblies of divine worship so as to express a reverence for God, and a content and satisfaction with that rank in which he has placed us."