Why? Because Coinbase are a well known big block sympathizers, they are essentially a Bitcoin bank and they would benefit the most if the network would become more centralized because of bigger blocks.
At first they were supporting SegWit2x, now they are pushing Bcash as a scaling solution, claiming that it's for the good of their customers who are suffering from high fees.
But in reality Coinbase is responsible for the current congestion, they never implemented batched transactions that reduce fees by 90%, they didn't implement SegWit that reduces fees by 50-75%.
This just shows that they don't really care about their customers, their real goal is to attack Bitcoin. Manipulating the Bcash price is a part of this attack.
Yeah some ulterior motive must be at play for such a chaotic event. There's no way a professional outfit would've pulled any of that. And there's no way they'd go all in on such an absolute shitcoin either. The concept may have merit, the execution is a flat out insult.
I've seen tweets recently about the wretchedness of their handling of transactions. Again, you don't do it that badly without a reason but maybe it's just a lack of time and competent staff.