Zig-Zag in pushover curve

[mohamedb_in@yahoo.fr]

Dear all
I am doing pushover analysis for a low-rise RC frame building using inelastic plastic-hinge force-based frame element (infrmFBPH) considering displacement control.
Can anyone tell what the possible reasons are for having a zig-zag in some parts of the pushover curve (see attached figure), and is this acceptable?
Thanks in advance.

https://image.noelshack.com/fichiers/20 ... lin-xx.jpg