Well... I know many people believes that the hexenbiest 'spirit' got into Juliette through Adalind. Since she turned into Adalind using the reverse spell she did to turn into Juliette, it is easy to assume that was the reason.
BUT... think with me: Nick made Adalind's powers go away. That said, Adalind was not an hexenbiest anymore. Then, she passed through the Contaminatio Ritualis and then got another hexenbiest 'spirit'. When Juliette passed through that spell to turn into Adalind to get Nick's powers back, if the hexenbiest 'spirit' was indeed from Adalind, shouldn't Adalind lose her powers to Juliette? Since she didn't, we can just assume that is another hexenbiest and where that hexenbiest came from? I can't really make sense of this. Unless hexenbiests can be made out of nothing. If it is another 'version' of Adalind's hexenbiest then why it is so powerful? Shouldn't Juliette have the same power as Adalind?
I remember when Monroe got kidnapped, when Juliette asked Rosalee about an ordinary person becoming an hexenbiest, she said that it is possible and generally they become even more powerful than natural/born/inherited hexenbiests. Maybe that is the reason? Or there is something else? What do you think?