Stupid Question

There are no stupid questions. Asking questions is how we all learned this stuff in the first place.

There are quite a number of things that can cause PH to rise. The most common is TA combined with aeration. Aeration allows CO2 to outgas, which raises the PH. If there was extra aeration, perhaps a particularly intense thunderstorm or more people swimming than usual, that could cause the PH to rise suddenly.
 
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