It doesn't say that. You're drawing your own conclusion from the score decrease. Also, I didn't downvote you.
The only information we are given is that the OP paid off a debt and the credit score went down.
If that was the OPs only long term debt being serviced, (credit cards don't count), the credit agency now has no proof you can CURRENTLY pay off a new debt. Meaning OP is a slightly higher risk.
Credit agency has no idea where the money came from that paid off the debt. It only knows that OP was regularly finding money somewhere, and that OP was putting that money toward debt as agreed. Did OP lose their job after paying off the debt and doesn't have income anymore? Did OP have someone else helping them pay that that person won't help in the future? The credit agency has no idea. It only knows that in the past they were able to service the debt, and today they have no way to measure if they can. So it is a slight increase in risk, meaning slight decrease in credit score.