There's a rule of thumb in chemistry that say that increasing the temperature of a reaction by 10 degC doubles the rate at which it proceeds, but the converse is true - if the temperature is reduced by 10 degrees, the reaction takes twice as long. 20 degC will slow it by a factor of four.
As your curing mastic is outside, this rule of thumb suggests that low overnight temperatures could have slowed down the curing reaction enough to give you an extended curing time.