Payments automatically getting applied to posted sales invoices


We are having issues with payments automatically getting applied to any posted sales invoices. This is only happening to two customers out of our 34. I've checked and re-checked the setting on Customer Card and everything is same as rest of our customer's Cards.

What exactly happens is,

Sales Invoice INV-12345 gets posted on 04/26/2017

Payment gets applied to the INV-12345 on 04/26/2017 even though we have not received any payment yet for this INV-12345.

How do we stop this automatic payment mechanism.

Please advise