If you look at Coinmarketcap figures, there is a steady secular drop in bitcoin market cap as opposed to alt coins.
People liquidated their holdings due to recent negative news around the bitcoins. At the moment, there is comparatively less demand for the bitcoins as people don't want to risk their money. Some of these people and new investors are shifted towards the alt-coins as they are providing good amount of profit than bitcoin. This could be one of the reason for steady drop in bitcoin's market cap. I believe this situation will be changed soon and bitcoin will top the market cap as previously.
There are thousands of different tokens, and 30% is a huuuuuge piece of crypto pie:) it is obvious that people diversity their portfolios, because bitcoin is good for long term deposit. And people tempt to earn here and now, so they go for short term trades.
Yes, those who are looking for the quick profit and don't wish to hold the coins for longer time are investing into the alt-coins. Currently, there has been lot of movement in the alt-coins which generates the opportunities for traders to make the profit.