Because it's the default in CKEditor configuration.
Apparently setting the config property
enter_br would fix this accoring to
However, the CKEditor documentation states that this setting is not recommended. http://docs.ckeditor.com/#!/api/CKEDITOR.config
Please file a feature here in order to have this as a configurable option: https://github.com/mendix/CKEditorForMendix/issues
config.enterMode = CKEDITOR.ENTER_BR // pressing the ENTER KEY input <br/> config.shiftEnterMode = CKEDITOR.ENTER_P; //pressing the SHIFT + ENTER KEYS input <p> config.autoParagraph = false; // stops automatic insertion of <p> on focus
Adding these to the config.js file in the widget fixes the issue