To add: Curling on ice used for other purposes too leads to shitty ice. The ice needs to be flat and level. Zambonis do not lead to level ice. Dual purpose rinks likely also don't shave the ice to ensure level and flatness. Likewise, gouges in the ice from skates causes unpredictability and some might compensate by over pebbling prior to playing. Also poorly/sparsely pebbled I've can lead to unpredictableness.
The club I curl at is strictly dedicated curling ice and most curlers there take care and pride in the ice and work to keep it clean and in to condition. But we are also open to introducing people and bringing new curlers in. Sometimes people don't wear ideal clothing. They will wear clothes that is linty/fuzzy. Some lint or fuzz gets in the ice and a slow curled stone goes over a piece in the ice which causes a lot of friction b pulling the stone and causing it to start curling the opposite way. That's what I often see. Another thing that sometimes messed with the ice and can cause negative curl is outside conditions. The building isn't perfectly sealed and there are doors to the exterior 3 feet from the outer sheets. If it's above freezing the warm air seeping in can cause the ice near the doors to be frosty or wet (or even colder than normal and harder to melt from some friction if it's extremely cold outside). Depending on conditions. This can mess with the friction between the ice and stone regardless of spin direction.
Probably gravity. Shaving the ice in the same pattern every time, or excessive traffic on the edges of the sheet probably cause uneven surfaces over time.
To add: Curling on ice used for other purposes too leads to shitty ice. The ice needs to be flat and level. Zambonis do not lead to level ice. Dual purpose rinks likely also don't shave the ice to ensure level and flatness. Likewise, gouges in the ice from skates causes unpredictability and some might compensate by over pebbling prior to playing. Also poorly/sparsely pebbled I've can lead to unpredictableness.