There is a thesis that the amount of new sign-ups to Coinbase is causing the general growth in the top 3 and due to bitcoins price people are thinking 'hey i can have 1 whole litecoin or ether' while they may also believe there is more room for growth given the price is lower for these two. Also, there was news of a blockchain research team in UBS announcing some exploratory using Ethereum.
Thank you, that made sense! I just put a few thousands into Litecoin and Etherium on top of what I already invested.. let's hope they will grow among the other currencies.