With the upcoming bitcoin fork on Aug 1, 2017. Why would all other coins be falling while Bitcoin is rising? Wouldn't one want to sell bitcoin to hide their coins in like LTC to weather the storm?
Several possible reasons come to mind:
People are actually confident that everything will go alright: this is the most unlikely scenario considering everything else is dumping which is the expected panic behavior.
Whales are keeping the price up until the deadline to draw people in: this is more likely I think. I still expect the price to drop considerably before the deadline.
People are selling their alts and moving to BTC massively to take advantage of the hard fork and get some Bitcoin Cash when it splits: I think this is also very likely and will explain the current behavior.
I personally contributed to the 3rd option.