In our experience, it'll be down to the pesky but very much needed SCA limit being reached meaning your money is safe.

Once you’ve made a number of contactless transactions that adds up to £300 it's likely your next contactless payment will decline. Nothing to worry about, this is just the system protecting your money and making sure you're still in possession of your wearable. You'll just need the physical Curve card and make a standard Chip and PIN transaction, this will reset your limits, satisfy the system and you're back off on your merry way.

Depending on the country you are in, there are different values that will trigger the Strong Customer Authentication and you can find the full list right here

If you know it's not SCA we would suggest that you speak with your card issuer/ check your online banking to see what happened.