There’s no way of knowing why the IRS changed your payment method other than by contacting them. But at least you know that your payment is coming.
You can see Coronavirus Assistance Impacts at the top of the IRS's Let Us Help You page for a phone number related to Economic Impact Payment questions, but they’re experiencing massive volume right now, and it may not be worth your time to stay on hold, since you know your payment is coming.
And if your payment is scheduled via mail, it’s not likely they can change your payment method, anyway.
The IRS’s Get My Payment page is the most reliable source to find the status of your stimulus payment.
[Edited 3/22/2021 | 8:28 am PST]