Well, I may ask how does this-- and many other things-- happen in my garden, but I usually know the answer.
Case in point, I found this shrub Honeysuckle growing right along side, almost inside and mixed up with a Rose-Of-Sharon. At first glance it would seem it happened overnight--that is to say, they do grow quick enough to make you think this, but it's not the case, of course. I missed it as a tiny seedling and it grew unheeded, untouched and very fast. By the time I found it, the Honeysuckle was already half a Glory tall and much more difficult to extract.
Easy to do...to miss it I mean. It's also easy to snip it in the bud and be done with it....if you are lucky enough to catch it as a seedling. *Sigh* that's my problem...never quite catch them in time. Hence why I find myself asking how does this happen?