14 陸奥の しのぶもぢずり 誰故に みだれ初めにし 我ならなくに (河原左大臣)
I chose this purely because it was one of the two left over from what everyone else chose. Turns out it was quite a good choice, because there was a lot of interesting information about it to be found. In fact, I’m pretty sure there’s loads that I still haven’t seen, but for now, here’s what I found:
Author
河原左大臣 was renowned for his courtly elegance and it is rumoured that he was used as a model for the hero in the Tale of Genji. Formerly the son of Emperor Saga, he has been made commoner and had to go by the name みなもと, just like Genji. He later became known as ‘the Riverbank Minister of the Left’ after building a huge mansion on the west bank of the Kamo River in Kyoto, in which he hosted poetry gatherings.
This poem is also the reason some believe him to have travelled all the way to Shinobu, in the northeast of Japan, for official business, where he fell in love with a local woman, which in turn delayed his return home. When he did have to leave, it supposedly inspired him to write this poem. Although this is very likely not the truth, the use of Shinobu became a famous 歌枕(うたまくら: oft-repeated descriptive epithets in poetry. Thanks Jisho).
Content
The exact meaning of the poem is still debated, as it can either be interpreted as the poet’s defence of his faithfulness to his wife, or as an expression of a secret love to someone else. In the latter case, he is actually shifting the blame from himself onto the lady in question for being too attractive. In the Heian period, a forbidden love meant being in love with another man’s wife or with a woman of much higher rank than one’s own.
As such, the ‘wild’ or ‘tangled’ pattern of the しのぶもぢずり represents the heart moved by love through no fault of oneself (われならなくに).
Grammar/Poetic Device things
The first two lines of this poem can be read as 序詞 (じょことば: ‘prefatory modifying statement of a waka, etc.’, courtesy of Jisho. Also related to 枕詞, which we’ve also seen before, but it is not restricted by the number of syllables.) to the following みだれそめにし. After he then goes on to ask who would be responsible for these feelings of his (誰故に), and eventually shifts the blame from himself in the last line, using a form of 倒置法 (とうちほう: word order inversion in a sentence). Grammatically, that line should sit before the third line; the open ended question inferring the blame lies with the other party, the woman. That grammatical and syntactic break after the fourth line is a poetic device called しくぎれ.
As with some of the other poems, we have yet more かけことば in this one.
しのぶ can actually refer to three different things in this poem: the actual location mentioned above, the plant (fern), or the verb meaning. In the latter case, the verb itself has different meanings as well: ‘to conceal’, ‘to endure’, or, if written with a different kanji, ‘to think longingly of’, hence the inferred meaning of ‘to love secretly’.
みだれ in みだれそめにし refers to being in disarray. そめ can either mean ‘to dye’ or ‘to begin’. The former is 縁語 (えんご: associated word), referring back to しのぶもぢずり.
Trivia
Shinobu in Michinoku, referenced in the poem, is now the city of Fukushima and was, among other things, famous for its pattered cloth dyed by しのぶもぢずり. Fern would be rubbed onto cloth to create a ‘wild’ or ‘tangled’ pattern.
A big rock, the もぢずり stone, lies in the ground in a small Kannon temple in the outskirts of Fukushima City; supposedly it was used to create the pattern by using it to rub the fern pattern onto cloth. It is also said that the woman left behind by みなもと visited that same temple 100 times, after which she was able to see his face in the stone like a mirror.
In the first story of ‘The Ise Stories’, this poem is referenced as well. In the story, a man was hunting in Kasuga, Nara, when he spied two sisters through a crack in a fence. He used his hunting cloak, which had theしのぶもぢずり pattern printed on it, to write a poem and send it to the sisters. Their answer was this poem, written by 河原左大臣.
もぢずり refers to a type of plant, called ねじばな in modern Japanese.

Sources
https://100poets.wordpress.com/2012/07/26/whos-fault-is-it-poem-number-14/
https://adblankestijn.blogspot.com/2016/04/hyakunin-isshu-one-hundred-poets-one_2.html
http://www.jlit.net/premodernlit/hyakunin-isshu/poems_11-20.html
https://honda-n2.com/honkoku-ogura-hyakunin-isshu-14
https://www.ogurasansou.co.jp/site/hyakunin/014.html
As always, please do add to or question all of this, chances are I’m wrong 