[chris (OP)]

Water under a lower pressure boils at a lower temperature because the lower pressure makes it easier for molecules to gain sufficient energy from the liquid to sever the intermolecular forces within the fluid and escape as a vapour. But what happens to the freezing point?

[Bored chemist]

At a reduced pressure the freezing point rises slightly. (At most, roughly 0.01 degrees C)
For most materials (those which contract on freezing) the freezing point would reduce .

It's an example of this
https://en.wikipedia.org/wiki/Le_Chatelier%27s_principle

[vhfpmr]

https://naeye.net/wp-content/uploads/2021/01/water_phase_diagram.jpg

[chris (OP)]

Dare I ask why this phenomenon occurs?

[Bored chemist]

Dare I ask why this phenomenon occurs?

Apparently.
And to an extent the answer is

https://en.wikipedia.org/wiki/Le_Chatelier%27s_principle

[alancalverd]

Fun news item this morning. Apparently University College London have made (or discovered)  a disordered  state of ice by milling the regular stuff into a glassy solid with a density of 1.0.