Science of Chocolate Seizing

I've made chocolate mousse and ice cream for years with grocery store chocolate with no difficulties. Twice, though, I've used 70% bittersweet chocolate, and it's seized up as I was tempering the egg yolks. Happily, the cream for the ice cream was warm, so I dumped the whole mess into the cream and saved it. Chocolate mousse -- I had to start over with Baker's chocolate, instead of Scharfenberger. I really don't believe it's my technique -- I've only had trouble with the high quality stuff. Could it be the chocolate, or have I coincidentally messed something up?