High-pressure areas are normally caused by a phenomenon called subsidence, meaning that as the air in the high cools it becomes denser and moves toward the ground. Pressure increases here because more air fills the space left from the low. Subsidence also evaporates most of the atmosphere's water vapor, so high-pressure systems are usually associated with clear skies and calm weather.

in the northern hemisphere, we experience rapid atmospheric cooling starting around the beginning of september as a consequence of the earth's tilt as it orbits the sun in a wobbly and imprecise manner, which is why we get two things near the end of the year:

1) a lot of high-pressure systems.
2) winter.

further, when you see high-pressure systems assert themselves earlier in the year, or linger later into the spring, the reason this is happening is that the sun isn't warming the atmosphere up enough, or even because the amount of solar energy hitting the earth on the tilt is decreasing in counter-intuitive ways. this is complicated, because it is non-linear. physicists call this an n-body problem, because you calculate it using the sum of the gravity exerted on the earth by the n bodies around it. there is no general solution to the n-body problem, and we are better off trying to measure this directly than solving it using numerical methods. the dynamic nature of the sun further contributes to the complexity in this system. climate scientists would refer to the fluctuations caused by the n-body problem, together with the fluctuations in the solar field, as the mechanics comprising the milankovitch theory.