"Unavoidable circumstances" refers to situations or events that cannot be prevented or avoided, no matter what actions are taken or precautions are in place. Full definition
If you trip has to be cut short or canceled because of sickness, severe weather, or other unavoidable circumstances. (johnnyjet.com)
This will save your money in case you will need to cancel your booking due to certain unavoidable circumstances such as bad weather, change of plan or medical reasons. (roamingpirates.com)
Interview rejection letters are often used to convey an unpleasant incident to applicants wherein the employer due to certain unavoidable circumstances, has to cancel the scheduled interview with the candidate. (template.net)