It's difficult to say but almost invariably this is when screen has been fitted before tray is sealed.
There is a gap behind shower screen profile that isn't sealed and any attempt to seal externally will just make a larger area for water to puddle and make it worse.

I understand that the thread is one year old, but I'll still provide a friendly casual reply to your post about the leak coming from the shower onto the skirting board.
Based on the watermarks on the skirting board, it does indicate a leakage issue that needs attention. While it's challenging to determine the exact source without further investigation, it's advisable to get a professional plumber involved to assess and fix the problem accurately.
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