Why does the Plutos atmosphere collapse

What causes Pluto's atmosphere to recede / collapse?

Keep in mind that until New Horizon arrives, we don't know much about Pluto's atmosphere. There seem to be a number of theories (the article I use as a source does not match the article linked in a comment) but our only way to observe the atmosphere so far has been on rare occultation events when Pluto is between a alternates observatory on earth and a relatively bright star with which the star can be viewed through the atmosphere. That can tell us a fair bit, but the first occultation observed was in 1988 and Pluto has an orbital period of about 248 Earth years, so we only saw the atmosphere near the perihelion (which happened in 1989). This in turn means that we have never seen the atmosphere at Apohelion and therefore do not know what it is doing there. We can model it, but we barely knew enough before New Horizon that I would expect the models to change as more data came in.

Even so, I use this article: Lifting the Veil on Pluto's Atmosphere, which provides a comprehensive overview of what we know from observation, what we suspect from modeling, and what we want to discover from New Horizon.

Pluto has an eccentric orbit: it ranges from a perihelion of 30 AU to an aphelion of 50 AU with temperature fluctuations from -220 degrees Celsius to -240 degrees Celsius. The low temperature is cold enough for the gases that make up the atmosphere to freeze and "snow" in solid form on the ground. It is unclear that this actually happened because we did not observe Pluto during aphelion. From the article I linked above:

Scientists originally thought that the vapors would freeze and fall back onto the surface of the dwarf planet when Pluto steps back from the sun and the temperature drops to around minus 240 ° C. Observations made as recently as 2013 and coordinated by the Portable High-Speed ​​OccultationTelescope group at multiple locations including the 0.9 m astrograph at Cerro Tololo Inter-American Observatory (CTIO) and the 1 m Liverpool Telescope in the Canary Islands, however, indicate Plutos The atmosphere does not collapse, but thickens.

Although Pluto's atmosphere becomes thicker and thinner as the sun orbits, it may never completely freeze and "collapse".

Michael Summers, New Horizons co-investigator and member of the atmospheric science thematic team, said it was too early to say whether Pluto's atmosphere will freeze or persist in its orbit. This makes sense because Pluto has only made one-tenth of an orbit around the Sun since it was first discovered in 1988.

To answer your question directly, as Pluto warms at the perihelion, the atmosphere grows as the gases warm and expand (or melt from solid form). In aphelion, the opposite occurs when the gases lose temperature and either freeze or simply become denser.

For fun (and to demonstrate a cold-reducing gas volume), here's a video of dipping a balloon in liquid nitrogen: