Causes are probably several including those you mention. I have not seen BWV standard studios disappear at 11 months out during the low to moderate 8 1/2 month season, but AKV value studios do quite a bit. The 11 month window (and 7) for the high season at WDW has really changed significantly in the last three years. Three years ago the inability to get something like BLT standard or BWV standard or boardwalk view studios at 11 months out was more of a fairly minor issue with only certain periods like early Dec being a real problem. You now have that risk for much of the high season.
In fact, if your typical time to go is October, the October use year has become somewhat of a liability rather than a plus for such rooms at 11 months out. That is because many are walking, starting in last week of September to get times for Columbus day weekend and the two weeks thereafter. Someone with an Oct use year cannot really start walking in Sep because he cannot use, for example, Oct 2017 points to reserve any time in Sep 2017 without first borrowing, so to use those points for walking, you have to start with an Oct 1, 2017 reservation, and Oct 1 and some days after can be gone before 11 months out, reserved by those starting reservations in Sep (in fact I actually saw that happen at 11 months out for this year, and then Oct 1, 2, 3, and 4 opened up again a little later but Oct 5 and after did not for Columbus Day time)