YouTuber Geogia Blainey explains:
Yes, boiling and freezing can occur at the same time!
We think of freezing and boiling as opposites, things that happen at very different temperatures. However, under conditions of low air pressure the two can occur together.
Substances can only be liquid in the presence of a gas to apply vapor pressure. If there is no pressure, or too little pressure, a substance will go straight from solid to gaseous phase as it warms. If you do this in an enclosed space the gas produced will build up, creating pressure as it does so, until a liquid phase is possible. If not enclosed, the gas will escape and the process can continue indefinitely.
The video is what happens if a liquid is placed in conditions where the temperature is right at the point where it freezes and the gas pressure is removed. With no pressure it can't stay liquid, but it is right on the cusp between boiling and freezing, so it does a little of both. Going from liquid to gas absorbs energy, while liquid to solid changes release it. This can keep the process in balance - if too much material boils the temperature will fall, shifting the balance in favor of freezing.