Recent action would suggest this market being heavily influenced by traders and institutional players shifting funds between Bitcoin and Ethereum causing spikes in one while the other stays still.
Observe when Bitcoin was ~$2500 and Ethereum in the $240 area. Bitcoin surged to new all time highs while Ethereum was left twiddling its thumbs...
The last few days the same can be said about Ethereum... it had a violent move to new all time highs breaking the $400 barrier while Bitcoin's move was less spectacular and even dipped...
I sense this may be the way things will shake out for some time... perhaps until Bitcoin sorts out its scaling issues. Folks are having a hard time which pony to ride off with into the sunset... so taking turns until there is a decided preference. Both will do well but the FOMO is intense!