What are causes of dog aggression?

We have had our Toy Poodle for 5 years , His name is oscar he used to be very playful as a puppy and about till 3 years of age . He loved to chase the cats and used to play chase with me and my friends :) But about 2 years ago i took him to my friends house , They had a Chihuahua and a Shepherd Mix both larger then him even the chi , He ate some of their dog food which was no problem he did it all the time when we were over there , well the Shepherd mix started growling at him i got off the couch and went to grab him , but the chihuahua tag him my friend pulled him off before he did to mich damage . He only bled a little and he stopped playing with me he just went on walks no playing of any kind and he was skiddish of other dogs , well he didn't start acting out of normal until now . The other day he bit me , giving me his food . And he growled at my granny that day too , and ended up biting her . Well today he bit a maintenance Man . on the leg it didn't go through as he had on blue jeans . We usually go on trips to petco to by him treats and food . But , now i'm starting to get scared to carry him anywhere . Could this be of his older age or a psych problem ? We are really wanting to get a another dog but , we have been very scared to bring a new puppy in , Especially a Great Dane puppy and that's what we were going to get .