A lot of it boils down to blood pressure. High blood pressure is a serious contributing factor to cardiovascular incidents (as well as a slew of other negative health risks), and getting a good night's sleep will help keep blood pressure down. This is also why the amount of heart attacks are up around 24% after daylight savings[1]; an hour less sleep means higher blood pressure means higher risk of heart attack (relative to any other 'normal' day).
I can definitely see how the same logic could apply to Mondays. Less sleep, more stress = higher blood pressure = higher risk of heart attacks.
[0] https://www.goodreads.com/book/show/34466963-why-we-sleep
There is no proof of this as to why. Only that it happens. For example:
* the hour of lost sleep does not happen on Monday, but on Sunday morning
* people could sleep in on Monday, then get hyper stressed that they are late
* type A personalities could get mad at an office of sluggish people
* people could get upset at everyone complaining about DST again, my blood pressure went up at your post!