"All water filters are effected by freezing. When water freezes it expands, and that can cause cracks, punctures, or other forms of failure. The simple solution to this problem is to disconnect the filter from the bladder and then suck out the water that remains inside. Or you can simply put the filter inside you warm sleeping bag."
Water expands when it freezes
If water is in an enclosed container that can't stretch enough, and the water freezes, it will expand the container which breaks it
With a hollow tube filter, if the entire filter is full of water and it freezes, the outside case will probably crack
If the hollow tubes are full of water but the rest of the filter is empty which you can do by shaking it out, and it freezes, the tubes will just stretch with no problem. When you backwash the filter, it stretches the tubes a little, including making the pores slightly bigger so any particles will be released and backflushed away.
Obviously, this logic would have to be tested.
Yeah, you can leave it in your sleeping bag or whatever, which is what I do, but over the life of the filter it would be so difficult to keep it from freezing every time. And it could have frozen once but you didn't notice.
Sawyer says that they don't know whether freezing will damage it, so just in case, if it freezes, throw it out and buy a new one. If it freezes, go ahead an use it for the rest of the trip.
All of this is very unsatisfying, especially if the Sawyer is actually freeze-proof
Here I go again, flogging another dead horse