I'm pretty sure what is happening. Windshields are actually 2 pieces of glass glued together with a flexable rubber type glue. This keeps the glass pieces together in an accident and can keep the occupant in the vehicle.
Anyway, if the glass edges are not sealed well, moisture lying in the bottom channel can work it's way between the glass. I assume extreme temperatures can aggrivate this.
I have no idea if this can be fixed or arrested. I can only guess removing the glass and applying heat may expell it. Then reseal the edges, or just reseal the edges to prevent it from getting worse.
You really need to talk to a glass expert, or perhaps one will be along here. Good luck.
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