What the Bible says about Niphal stem
(From Forerunner Commentary)
This verse presents us with a clear example in which anah is not in the niphal stem (reflexive case), but clarity is achieved by explanation: "But as for me, when they were sick, my clothing was sackcloth; I humbled [anah] myself with fasting." In this case, the self-imposed affliction or humbling is by means of fasting. Ezra 8:21 is another example: "Then I proclaimed a fast there at the river of Ahava, that we might humble [anah] ourselves before our God, to seek from Him the right way for us and our little ones and all our possessions." Again, anah is not in the niphal stem, but the rest of the verse explains that the humbling comes through fasting.
John W. Ritenbaugh
Pride, Humility, and the Day of Atonement