Usually - Alkalinity cannot increase 'on its own'.

Does alkalinity go up on its own?

According to multiple articles and posts from @Randy Holmes-Farley - its close to impossible for alkalinity in 'increase on its own'. If you leave it it will gradually decrease.

What causes alkalinity to rise?

One of the main reasons pool alkalinity can get too high is when you have added too much sodium bicarbonate (baking soda) when you were trying to raise it. ... Another factor that often causes alkalinity levels to get too high is your water source.