Negative Float in Primavera P6 program

My P6 Baseline schedule is showing negative floats (up to -4 days) for few activities. This schedule does not have any Constraints and there is no "Must Finish by" date populated in Date tab of project details. But this project has Mutiple calendars, like 5 days with Day shift (8am to 5pm), 5 days with Night shift (11pm to 7am), 24 oursx7day. Some calendars are blocked with public holidays, RDOs (Rostered day off) X'Mas/New year break for 2 weeks etc. Not sure if this is causing the negative float. I am looking for an explanation for getting negative float