When you get an adjustment, your vertebrae are being moved slightly. Your muscles have to adapt to the movement of the bone, so they may end up lengthening or shortening slightly, which can lead to soreness.
The soreness is related to the movement of the bones and not to the pressure utilized by the chiropractor. It can also be due to the release of the tension in your muscles, similar to when you work out and are sore as your muscles repair themselves. As your body gets used to being in proper alignment, the muscles will adjust accordingly and stop being sore.
The more frequently you get adjusted, the faster your body will get used to being adjusted and the longer it will “hold” the adjustment, which will reduce or eliminate the post-adjustment soreness. If you have long-lasting or extreme soreness or continue to be sore after many adjustments, talk to your chiropractor or orthopedic surgeon about exploring other possible causes for this. https://www.nhs.uk/conditions/chiropractic/