This is happening because you might have accidentally enabled checklist option. Checklist option allows you to tick off the box that is added at the beginning of the line. So that you can track tasks completed.
Watch this video to know how the feature works:
Creating a checklist inside a sticky note that uses the Standard editor in Notezilla
Creating a checklist inside a sticky note that uses the Markdown editor in Notezilla .
To stop this, you can select the 'Remove checklist' option from the bottom Markdown toolbar.