Date constraint on task without dependency

A date constraint is visualized by a vertical line at the beginning of a task. This article tells me that a date constraint is only created when a dependency is added between two tasks.

I have tasks in my Gantt chart with no dependencies but they do have such vertical line. What does this mean? 

help.wrike.com/hc/en-us/articles/209604249-Date-Constraints-on-the-Gantt-Chart?flash_digest=7796a895049edd54192cb4e7037742638cee0589