I think number of BTC wallets holding large amount of coins correlates with number of new exchanges as people don't like using older exchanges because their favorite coin can't be listed on them, so they start new exchanges just for listing those coins... Eventually community of one coin will start using the same exchange and that will exponentially increase number of coins in the exchange wallet... Another factor is the developers of new coins who cash out their altcoins as BTC and keep some of them waiting for future exchange listing fees.