Its not the piracetam that does it (although it has been occasionally reported). Its the fact that piracetam potentiates the drug so much. It potentiates both the benefits and the negatives pretty uniformly (besides the possible neuroprotective qualities) as it potentiates the activity of the drug as well.
Piracetam and MDMA is a weird case. I experimented with it a number of times and it definitely potentiates it but it also increased the side effects, namely the tachycardia.
I wasn't the only one to report it. The problem is that it is hard to say what actually caused the increase in the heart rate, the piracetam, the MDMA, or the increased sensitivity to MDMA that the piracetam induced.