Every chamber has a number of modes. Modes are resonances of an enclosed empty space. A small chamber like the back of a headphone has these modes above 5kHz. A medium one, like a loudspeaker box will have them lower, around 500Hz. A big one like your room will have modes around 50Hz.

These resonances at the back of an earcup will stop the diaphragm from moving at certain frequencies and boost it at other, creating the zigzagging response that you see in measurements.

IF you want to get rid of them, you can damp them with some sound absorbent material or with tuned resonant chambers. At least this is the way loudspeakers are made. Headphone earcups are usually too small for absorption so the only way we deal with this problem is by venting them. That's why we have open headphones.

Bass in headphones and loudspeakers is created by diaphragm excursion. The diaphragm rests on an air spring created by the closed chamber behind it. In headphones there is also a front chamber and the eardrum, but let's ignore it. The air spring opposes high excursion of a diaphragm. In practice it reduces bass extension, but amplifies the frequencies just above that. This is why you rarely get deep bass with closed headphones. They often have a bump around 100Hz and then roll off like it's nobody's business.