Differential cooling in the upper layers of the atmosphere is making it increasingly unstable, and forming convective storms.
A look at mid tropospheric temps vs the lower level temps reveals that globally, the atmosphere is more unstable because of the deep level cooling that has taken place! Its that simple. The cooling in the low levels has not yet caught up to what is going on aloft, and because of that, the way to balance that out is in the form of more atmospheric fights.
h/t to Marc Morano