Austrian Airlines review by Rodica

We had a very unacceptable situation happen at boarding. My family and I bought return tickets (Chisinau-Vienna) 2 months before the trip. At boarding we were told that the company sold more tickets than places on the plane so we didn't get seats on the plane. Why is there a possibility when a person buys rather expensive ticket (€314 per return ticket) that person has a chance of not getting on the flight he/she paid for? The person at boarding was very rude to us, raising the tone and couldn't explain the problem properly, continuously saying "It is not my problem", which seemed very unprofessional to be said by a representant of the company. My family and I will never use the services of Austrian Airlines again.