I wasn't sure if I was the only one thinking that. I loathe Ted Cruz though... I thought it was just my own bias.
I think it's because we usually only see a narrow range of emotion from Trump's typical performances and appearances. So when he goes outside of that range, it's a little bit more impactful.
Kinda like how a comedy changing tone to deliver a big, dramatic moment can be many times more impactful than a drama delivering a dramatic moment. It's a great device, so long as it isn't over-used.