There are probably a lot of different theories floating around.
My pet theory is that all else equal additional outside demand (meaning purchased using outside fiat wealth) for BitAssets should raise the BTS price (assuming latent short demand exists). Of course if there is a change in sentiment for the future of BTS while there is simultaneously outside wealth pouring in, then the net effect could still be a net decline in BTS price according to this theory. Nothing surprising there.
Furthermore, my theory claims that all else equal if the demand for BitAssets is coming not from outside fiat but from BTS holders in the internal exchange, then the net effect on BTS is zero assuming latent short demand exists and if latent short demand disappears then the net effect could actually become a downward pressure on its price.
Arhag and I have had some involved discussion around these impacts, although time permitting (for me) we still have some points to resolve. Suffice to say, there is no mechanism I am aware of whereby more bitAssets should lead to BTS price decline. As arhag says, and a point we found agreement on, was that if the latent demand for BTS were unchanged, bitAsset creation (which provides additional lending to BTS buyers) ought to lead to increased bid prices for BTS as the stock of BTS ends up getting more and more concentrated in the hands of the most bullish cohort of investors. This is similar to the way a property market might get bid up when there is easy bank lending available. As arhag also said, there are other factors in the market that will also affect the price at any time.
However, in my personal opinion, should this cohort attempt to exit these positions, or if bitAsset demand reduces to force this (think credit contraction), then price needs to fall back by exactly the same extent, and no net profits are made. So the key ( for a sustained value increase that benefits everybody) is really for the overall market to be willing to attribute a higher valuation
to BTS as a result of the bitAsset demand and other return prospects for BTS. To be fair, this is a minority view and hotly debated (and still open to review on my part depending on ongoing discussions with arhag etc).