Anyone that's had dinner with my family would know why! (Thanks, I'll be here all week...)
More seriously, what a not-very-helpful article: "well, it could be these things that you might have already guessed, or shoveling snow, or...". The question of "why" ever got answered, IMO, just a lot of things that kinda make sense, but with no real data behind it.
There's an important one in there that is actually relevant: being cold. I even know it causes my blood pressure to rise but I never thought of that as a risk, just noise that was messing with trying to see how my blood pressure varied over the day.
I think family related stress and conflict could definitely explain some of it. I'm not sure how to design an experiment to test though. I'm not sure how to quantify "family related stress."
We have millions of fitness trackers being used worldwide. You could do an opt-in research study that would just compare your data before the holidays (baseline) and during for:
