Blockchain.info's
mempool transaction count is showing that there is very little to no congestion. Furthermore,
bitcoinfees.21.co is showing that even transactions that have fees of under 20 satoshi/byte are usually getting confirmed.
Coinbase was still sending transactions comfortably when blockchain.info's mempool transaction count was over 100,000 and sending fees of 100 satoshi/byte wouldn't get confirmations for weeks.
Considering that Coinbase actually sets people's fees for them and that their customers almost always got confirmations using their dynamic fees system during that period of real congestion, I would say that Coinbase are trying to steal your BCH.
Several other people, on here and Reddit, have been complaining about Coinbase withdrawals for the last couple of days. It's obvious that even if it was about network congestion, Coinbase's system would just make people pay higher fees instead of stopping withdrawals altogether.
The only non-conspiracy scenario I can think of is that Coinbase is not capable of handling the pressure of all these withdrawals at once.