From: http://www.mosaicprojects.com.au/PDF/Schedule_Calculations.pdf

Time Analysis Calculations - Complex 

• The single link calculations above remain constant. However, where there are more than one preceding or succeeding links, or constraints have been imposed, the calculations need to take  account of the different potential outcomes. The consistent policy is to use the ‘worst case’  option. This means using the latest dates in any forward pass calculation and the earliest date  in any backward pass calculation. The activity duration for all normal activities remains  fixed. 

• Constraints override schedule calculations in the following circumstances. 

As you can see negative float is the result of backward pass computations under constraints when the software overrides traditional computation of late dates. Do not forget, depending on the software, target finish of the whole job might be among the constraints but not specific of any activity, will be automatically applied to watever activity ends at this date when you have multiple open nodes. 

Same as you can have different float values under a WBS you can have different negative float values under same WBS.

To understand negative float on a specific activity I suggest filtering or tracking activities linked to the activity under investigation. The methodology will vary among the software and at times a filter by lag value will be enough to identify the chain(s).

Most probably you will find activities with -13 days float are linked across the WBS and share a common constraint that is driving the backward pass computation. Be aware that use of different calendars might also allow for different float values across any chain. 

Under resource leveled networks you might need to expose resource dependencies, if using Spider Project the functionality is embedded within the software while if using P6 you might need to use Ron Winter Logic-League and temporarily copy the links into P6, links you will have to remove for further resource leveling runs be correct.

http://www.scheduleanalyzer.com/ll_resource.htm

Most software always apply the date constraints to float calculations while others such as Spider Project provides the float calculations with and without applying constraints to the backward pass. This add visibility and understanding.

You can also look for Longest Path Theory though I believe if using P6 under resource leveling the computations might be hidden, maybe because the computations are not applicable, perhaps similar to computations for resource leveled float that are different to traditional CPM calculation without resource leveling.

http://scheduleanalyzer.com/sa_long_theory.htm

Best Regards,

Rafael