This has been happening to a lot of people lately (for at least the last 3 months) and, of course, no one is happy about it. There's a whole long thread about it. You can download Google Wallet to your watch and reprogram the home key to open it on a double tap. You will have to make it your default wallet, but you can switch back to Samsung Pay if they ever decide to investigate why this is happening.
Don't believe the "solved" tag - the moderators put that there and close the thread whenever they add content. It's their way of saying there's too much discussion. It is not solved until Samsung makes it not happen again.
