Does a vascular occlusion from filler happen because of bad technique?

Hello Cathy. The answer to your question is yes it can.  Injecting retrograde (pulling out the needle while applying pressure), aspirating before injecting and injecting more superficial or deeper than major blood vessels can help to decrease the chances of an occlusion, but they canhappen to even the best injectors.

Vascular occlusions can happen to even the most experienced injectors!  
Experienced injectors should know the anatomy, where vessels are located, when to go superficial vs. deep, etc., but even with this knowledge, it can still happen.  Not everyone's anatomy is exactly the same.  The most important thing is that you are going to a properly trained and qualified injector who knows how to properly handle and treat and event like this, should it occur.