What happens to the water inside the plastic bag after leaving it in the freezer overnight is the water freezes and then turns to ice.
The cold temperatures of the freezer causes the water inside the plastic bag to freeze because the cold temperatures of the freezer causes the water molecules in the water to slow down and lose their energy.
The slowing down and loss of energy of the water molecules allows them to form a rigid structure that is known as ice.
The reason why water turns to slush is because the water was supercooled.
When the water is supercooled it means the water was cooled below it's freezing point but without actually freezing the water.
Then a disturbance such as shaking the bottle or container or then pouring the water then triggers the ice crystals to form in the water and makes a slushy consistency.
Water turning to slush is a common phenomenon where the very pure water is able to remain liquid even when it's below it's freezing point.
That is until the waters nucleation point has been introduced and causes a rapid crystallization process.
Also as supercooled water freezes when you open it or tap it, it also warms the rest of the water up.
The heating might only allow 10 to 20 percent of the water to freeze and that also accounts for the slush being in the bottle instead of it being a solid chunk.
Slush is basically just a consistent mixture of frozen water and liquid water.
Slush can occur when the temperature is rising or falling, just like boiling water takes a long time, the freezing water also takes a long time as it takes a lot of energy to change the water between the states of matter which include gas, liquid and solids.
Shaking cold water turns the water to slush as the shaking of the water triggers nucleation in a phenomenon called supercooling.
Shaking the cold water and particularly water which has been chilled below it's freezing point of 32 F without freezing can instantly turn the water to slush due to the nucleation process.