Hi @PooJAN,
Without insight into the tickets or instance, this may be quite hard to answer.
I would however say a good place to start is to open one of the tickets where the state has been changed and view the history for that ticket.
This should provide some insight into when the ticket was updated and importantly which user performed the update. (Filter the history by state change)
This will let you know if it was a 'system' user or not meaning whether it was a system change (Script, Business Rule, Schedule, Workflow etc) or a specific change by a user/customer.
Additionally, check this update against the other tickets that meet the use case and look for a pattern or similarity.
It could be that a script for example has been run and inadvertently updated these tickets (Just guessing to help your thought process - as I said, it's quite hard to diagnose without more specific info.)
To help others (or for me to help you more directly), please mark this response correct by clicking on Accept as Solution and/or Kudos.
Thanks, Robbie
