Could be a bad batch or maybe high/tense action or rough saddle. I had my Kramer V break
two Ds in a row and it was due to the brindge saddle getting too sharp from the string
riding up and down on it. Filed it down a bit and was good to go. In your case since it
was on two different guitar I'd probably attribute it to a bad string batch first, try a
different brand just to see what happens. If both guitars have the same kind of bridge and
your playing style is heavy on the D with a lot of vibrato and pull offs it could also
just be your playing style.