That fact by itself might not be cause for alarm, but the underlying cause -- a large hashrate drop over a short period of time -- might be a signal that people are abandoning the network. Emphasize "might".
https://blockchain.info/charts/hash-rate
There is more information about the relative hash rates for both chains at http://fork.lol.
Secondly, as transaction fees are a supply and demand system, demand for transactions on BTC (tx demand on BCH is negligible) can shift that profitability ratio simply by paying higher fees. AFAIK miners aren't setup to switch back and forth on a block-by-block basis yet, but I wouldn't be surprised if they start to do that in the future.