Causing is probably too strong a term, but it can be exacerbating it.
If I am booking for myself, I am limited to times when I, personally, might want to take a vacation. If I am booking a spec rental, I am instead looking for times when lots of other people might want to take a vacation. And, as I wrote above, I am incentivized to make this booking as soon as I possibly can---after all, I know when the popular dates are much more than a mere 11 months in advance. That might also be sooner than I would know when e.g. my vacation is, or when the kids' soccer schedules are firmed up, or whatever.
So, it is entirely possible that the existence of a well-known spec rental market is skewing the existing "organic" booking patterns for popular periods earlier in time.
That doesn't change the fact that it is the OP's responsibility to book their own vacation as early as they might have. And given that they travel regularly during Jersey Week, this booking would have been foreseeable for them. That will be a live-and-learn experience for the OP I suspect. But, that too is exacerbating the problem. If a shift to earlier bookings then moves the "organic" booking patterns, then it compounds itself.
So, I guess where I sit both sides of this discussion are correct: it is the OP's responsibility to book as early as they can, AND the existence of the spec rental market might be making that more important than it otherwise would be.