Why are my mesenchymal stem cells expressing CD34?

I've been growing a number of MSC samples without any problem for a while. I've just analysed the phenotype on one of the samples and found that less then 95% of the population are expressing CD73, CD90 or CD105, and a worryingly high percentage of the population is expressing CD34. I have compared the sample at three different passages, P1, P3 and P6 (ages are 41 days, 57 days and 88 days respectivly) and the CD34 expression apprears at P3 (with 7% expression) and increases at P6 (to 46% expression).
During culture, there were no signs that these cells were not MSC, their morphology was spot on, their growth and adhesion were fine.
I have no idea how these possibly hematopoietic cells have appeared in my culture, can anyone help?
If it's helpful, they are growing in MSC Growth Medium from lonza and have been obtained from OA hip samples.