These fish are native to the tropical freshwater environment, specifically to South America:
ADW: Pygocentrus nattereri: Information
According to this link,
Pygocentrus nattereri are one of the most widespread breeds of piranha in the tropical aquarium-fish trade, and while they are banned in some countries they still manage to slip in (not sure of their legal status in Bulgaria). So, the release of piranhas from aquarium stocks is the most likely source for them being found in this lake (I don't think global warming can be blamed for this one
).
The release or disposal of live aquarium fish happens from time to time, but the exact reasons in this case are uncertain (and therefore the "mafia" speculation is inevitable). For example, this species of piranha has been released into freshwater bodies far outside of their native habitat in many different parts of North America, and according to the above link "
When a piranha is found in a lake, many state agencies use the chemical rotenone to kill the fishes."
But regardless of how they reached this lake, being a tropical Amazonian species, they probably could not survive too long in the wild in the case of a Bulgaria lake (we could hope). However, since they were previously found in September 2007 and now again in June 2008 they have either found a way to survive outside of tropical conditions or alternatively this would mean multiple (or repeated) release of the fish...