Probably just got used to being on its own, having its own territory etc. While particular species of fish might be considered non aggressive, every individual is different.
How big is the tank? Gold fish get big, live a long time, make a lot of waste. They need a lot of space to grow properly. To accomodate 5 goldfish you probably need a 75 gallon tank and filtration suitable for 150 gallons.
Do you understand the nitrogen cycle? Your tank has enough beneficial bacteria to support the single fish it had. Suddenly introducing 5x the bioload it has been supporting will mean your cycle has to catch up, a lot. If you were to test your water over the upcoming days and weeks you would see build up of ammonia and nitrite that could quickly get into toxic levels if not controlled through regular (daily) water changes until your cycle caught up and could deal with it. Thats assuming your tank/filtration can ultimately cope with with the 5 big, messy, fish you have.