So, hear me out... what if one of the reasons Morgan le Fay went so far as to assassinate her husband Urien is because he broke a taboo Morgan imposed on him, perhaps even three times?
In folklore, there is a recurring trope of the "Fairy Bride" - where a man, through various circumstances depending on the tale, manages to land himself a fairy woman as a wife. However, a stipulation is made that so as long as the man doesn't break a certain condition - such as "Don't strike three blows on the wife" or "Don't gaze upon the wife while she's giving birth" - the marriage would be maintained. Inevitably in these sorts of stories, the man ultimately breaks the taboo, thus the marriage is cancelled and the fairy wife magically leaves him forever.
Given that Morgan is the archetypal Fairy Queen in literature and the fact that, in most narratives, Morgan didn't really choose to marry Urien of her own free will, it is believable that Morgan would try to impose a condition or a set of conditions on Urien so that she could feel comfortable with the union. I could just imagine Morgan cooking up a set of rules so vaguely worded, Urien would break them invariably, one way or another.
This could serve as an explanation for why Morgan waited so long to enact her assassination attempt that, by the time it happens, Ywain is already a young man. Perhaps, in the beginning, Morgan was just "fine" with being hitched to a man she didn't love, but was slowly losing patience over time, with each violation of the taboo, intentionally or unintentionally. And then Urien breaks the last straw just recently, so now Morgan has the excuse to enact her murder plot of Urien (and Arthur, the person who brokered the marriage in the first place)















