Pittosporum Silver Sheen

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I have these plants around my pool area. Over the last 3 months, some of them have started to shed and die from the bottom, you can see in the photos where they look bare and the leaves start to brown before falling off. In the past some plants have died totally and quickly, within 1-2 weeks of showing first signs of struggling, where as these continue to thrive and grow albeit the leaves at the bottom are falling off

Any idea on what could be the cause and best course of action? They do get wet with water from the pool given their proximity to the pool

Fairygirl

If there are chemicals in the water, that could be the reason.
However, it will also depend on what they're planted in, and your climate etc. Not having enough suitable soil for the root system could be a reason. Are they in the ground? There doesn't look as if there's a lot of room for them.
Not all pittosporums are reliably hardy in the UK, so your location will also be a factor.
I don't think you're in the UK though, judging by the photo. 
Extremes of temps can see them off, but it could simply be a combination of several things. 

Joyce Goldenlily

Looking at your pictures and the space at the soil level, I am guessing there is virtually no water for the trees. With the concrete paths, gravel chippings, fence posts, and lack of root space because although the pool is above ground, it must be standing on some kind of level base, and with the closeness of the buildings and pool no rainwater will be reaching the tree roots. I think your trees could be dying of a lack of water. The weather looks very hot in the pic.

bédé

Looking at the area of damage, I would focus on chlorine/water splashing.