I agree that it is extremely confusing to people especially when trying to calculate profits and losses. In my experience as Bitcoin goes up in price the alt-coin markets shrink for a few reasons:

• Prices are adjusting in fiat to compensate for Bitcoin's rise in price
• People see their coin losing value and sell off to chase something more profitable
• Once the price starts to really tank (in BTC) people will begin to panic sell out of either fear or an attempt to re-position their investment
• Many investors cash out into BTC in order to take a profit or to hedge against the price contractions only to buy back in at a cheaper BTC price

Those are just a few reasons that I could think of, mostly because I've done all of those things myself haha.