mg: "My personal dislike is when people intentionally write songs to well-known tunes"
Well, I think you'll find that this has been going on since the dawn of time. There are more people in the world who like writing lyrics than tunes, plus in the days before recording more people who could read words than scores. Hence why so many songs (and hymns) were circulated in broadside and other printed form with just a suggestion for a well-knonw tune.
I agree it can be disconcerting when you hear a song with what seems _to you_ to be the 'wrong' tune, but it's probably the _right_ tune to someone else. In fact the tradition is so full of instances of shared melody that it's often impossible to marry lyric and tune with any authority.
Is this what you mean by "There are cases where songs have just jumped onto tunes"? If so then that sounds close to the idea that songs write themselves, or appear by come sort of community osmosis.
Songs don't just jump onto tunes. People choose to sing the words to a new melody on purpose, either because they only have the words and don't know the prior tune, or because they think the words and tune don't fit the old tune very well (perhaps because someone else did a poor job of 'jumping'), or they only have fragments of one or the other, or they want to tell a new story and decide for whatever reason (including ease of learning by others) to borrow an existing melody.
Just for the record I have no difficulty making new tunes myself (I suffer from a kind of compositional Tourettes, in fact), and have just as often written new tunes to old words as vice versa. My reasons for using only trad tunes in Jersey Jacques were threefold. I) The brief includes a secondary use in schools - and I wanted to help show the rich legacy of material out there . 2) The show is historically accurate, so using contemporaneous tunes helps to provide authenticity. 3) Not all the players will be music readers and I won't be there to teach them, so starting with at least some well-known tunes will make it more likely that people will sign up and have fun.
I suspect people who have 'jumped' songs in the past probably had similar motives.