It's probably miners switching between the two depending on which is more profitable at the time.

I'd guess that it is large pools of miners switching over together.

the difficulty of BCH is about 35% that of BTC. so blocks are generated faster if given the same hashrate. That makes BCH profitable to mine even at a fraction of the market price.

This flip of "profitability" has miners chasing the most $/second. Mining pools even have automatic switches that change networks depending upon the current network state.

The original difficulty algorithm is very slow, and the BCH "EDA" difficulty algorithm is predictably chaotic in a way that guarantees these immediate extremes of profitability vs unprofitability. It is being patched with a stable 144 block average presently (the next block at time of writing), at which point the swings will not be as extreme.

The reason it started around Oct. is because the price of BTC had historically made BCH unprofitable at almost any difficulty, but a positive BCH price swing has changed that.