904 Kicked into reverse by itself!!!!

Today I was doing some timing work on my 318 which is mated to a 904 transmission. I finally got the motor started, and at that point I got out of the driver's seat and walked around to the front of the car. After only running for about a minute, the car jumped into reverse by itself! I luckily saved the car from hitting anything! I want to know if anyone would have any idea as to why the car would do such a thing. Any reccomendations on how to handle the transmission after today? The car is a 1966 Plymouth Barracuda. Thanks FABO members!