1) Profitability. Quite a few folks chase the most profitable coin.
2) ETH seems to have finally hit the "2GB cards can't handle the DAG file size" barrier, which probably forced some of those rigs into something else - like ZEC - that doesn't have that limit.
3) People keep building more and more new rigs, since there are a few GPU mineable coins with good profitability right now - and ZEC is one of those.
ETH difficulty has ALSO been climbing a fair bit this past week despite 2) above and the sliding ETH price.