The upcoming hardfork on the 25th this month might play a role in this matter. In any case, Bitcoin is about to hit a new ATH. Previous ATH was on september the 2nd with a value of $4975 and market cap of over $82 billion. Currently we're looking at a price around $4900 and a market cap of over $81 billion. I took the mentioned numbers from coinmarketcap.com btw.
This is likely the biggest factor at the moment. The FOMO (fear of missing out) from the potential 'dividend' of the split coin is making a lot of people switch from altcoins into BTC. You can see this suppressing the prices of various alts right now. There's probably a bit of fiat being moved from the sidelines as well. It's similar to what happened leading up to the August 1st BCC/BCH split. It will be very interesting to see if we get the same follow through after the split this time as that could lead us up to $6000 or above.