The speculation which we are debating comes from two interlinked concepts.
The first is rolling news, which must bed 'saying something' all the time, even if there is nothing to say. The truth is, as Naomi and others have pointed out several times, that no-one knows why this man acted as he did, but that won't fill news bulletins, hence the speculation, which gives the news media something to pretend to talk about, even though it is actually meaningless.
The second is something I have advised several times - which is the need for humanity to find reasons for massive events in the world.
Our minds are unhappy with the vague (but probably entirely realistic) notion that 'things just happen because they happen and there is no point looking for a reason because there is no reason to be found', so we look to find solid ground because that comforts us and enables us to move on.
Step forward the conspiracy theory which is the entirely natural result of the concept explained above.
If there is no real reason for something, let's invent one, however bizarre and far-fetched it may seem, it still beats the alternative, simply accepting that 'things happen, and that's it...' which is the reality of the world we live in.