(1) Explain your problem, don't simply post "This isn't working". What were you doing when you faced the problem? What have you tried to resolve - did you look for a solution using "Search" ? Has it happened just once or several times?
(2) It's also good to get feedback when a solution is found, return to the original post to explain how it was resolved so that more people can also use the results.
It looks a result of Out-Of-Sequence activities. P6 default calculation of relationship lag is based on "Predecessors Calendar" which can be change on Scheduling Options Calendar for scheduling relationship lag to:
1. Predecessors Activity Calendar
2. Successors Activity Calendar
3. 24hr Calendar
4. Project Default Calendar.
You can check which of this option is set on your Scheduling Options so you can identify what is pushing the activity and why? specialy on Out of Sequence conditions.