Skype automatically answers calls on Android due to a bug, but a fix is in the works
as far as January, probably soon after the developers updated the app. Sadly, it appears that the issue now affects even more users, as we're seeing lots of recent reports posted on Microsoft's support forum.
For the time being, there's no workaround to prevent Skype from answering calls that you want to ignore, so the only way to avoid the issue is to uninstall the app until Microsoft fixes it. Not even having the “Answer incoming calls automatically” option disabled will not stop Skype from doing just that, AndroidPolice reports.
The good news is Microsoft has already identified the issue and managed to patch it, but only in the latest Skype preview app. If you want to continue to use the app without being affected by the bug, you can download the beta until the final version gets updated later this month.