To have some branches falling would not necessarily be concerning, as that is not unusual with hackberries.  However, your observation of rotten branches suggests that a little more investigation is needed.  While hackberry is pretty tolerant of many conditions, the stress of periods of drought could play a role.  I’m including several references below, one of which shows some possible problems that could affect hackberries.  Compare this with what you have been seeing.  You may need to do some close up inspection of the tree and fallen branches.  You could also consider hiring a certified arborist to come to your site and evaluate your trees and their environment to better understand what may be affecting them.  For your reference as well, the University of Minnesota does have a plant disease clinic should you have concerns about a disease with this or other trees in the future.  I’m including information below about this and their services. 
https://extension.umn.edu/trees-and-shrubs/common-hackberry
https://hles.unl.edu/hackberry-0
https://apps.extension.umn.edu/garden/diagnose/plant/deciduous/hackberry/branchesdead.html
https://extension.umn.edu/planting-and-growing-guides/how-hire-tree-care-professional
https://pdc.umn.edu/about