Airlines are far from perfect. They overbook flights and sometimes have to ask people leave and pay them for the inconvenience. My wife and I once got $1000 a piece and a hotel and food voucher to volunteer to take a flight the next day on a layover in Atlanta.
As far as your particular situation, the number one rule of using a third party portal to book flights or hotels is - don’t.
I understand that Iceland Air is not a transfer partner of Chase. But even in that case, I would just wait to use my points until I could use a transfer partner.
On the earning side if paying cash, the difference between 2x/3x points when booking directly and 5x when going through the portal just isn’t worth the risk.
Or they can just haul you forcefully from the flight you paid for:
https://en.wikipedia.org/wiki/2017_United_Express_passenger_...
I (unwisely) tried to purchase an Icelandair ticket via the Chase travel portal. I would get a reservation number, go buy seats on Icelandair's website, and a few days later the entire reservation would vanish into the ether. Rinse and repeat 3x.
I can't remember the exact verbiage, but basically tickets can be "reserved" and "booked". One means the ticket is allocated, and one means the ticket is actually paid for. I eventually sat on the phone with an executive support person as they booked the ticket and got it all the way through. It turns out Chase reserves a ticket on an airline but as an SLA of ~3 days to actually pay for the ticket. Icelandair's requires a ticket to be paid with in 24 hours, so it was timing out.