Most likely it was from site injections. Anytime you deposit extra fluid in a muscle through an injection, it increases the internal pressure that the muscle is subjected to. Inject in the wrong spot or too frequently and then when you get in the gym and start pumping blood in the muscle the volume becomes too much and the muscle tears in order to relieve the pressure it is under. Fluid, oil and blood have to exit the muscle someway very fast and a partial tear is normally how it happens.
Don't fool yourself. Constantly injecting anything in the muscle will weaken it and form scar tissue.