Excessive heat can cause them to stop laying eggs. I think the mechanism is that they don't drink enough or don't eat enough, and then their bodies don't have what they need, but I'm not positive about that.
Cold weather will usually not affect egg laying, as long as they eat and drink enough. So I'm not surprised that they did keep laying through last winter. Frozen water will leave them thirsty, and then they won't eat either, and that definitely can have an effect on laying. The short days of winter have a much bigger effect than the actual cold temperatures. That tends to affect older hens more than pullets. For first-year pullets, if they start laying at all, they will often continue all winter, despite the short days.
