Because the Devil makes them do it!
It's a matter of 'Free Association' - someone spots an unrelated detail in a post, and then feels compelled to post a comment about it. Then someone replies to that post ... and then someone else ... and pretty soon we have a case of 'Thread Incursion'.
(Case in point: Let's see how long it is before someone starts a sub-thread based entirely upon the first line of this post...)
_________________
The mere fact that science may not yet adequately explain an object, event, or experience does not mean the immediate explanation should automatically default to a conspiratorial, extraterrestrial, paranormal, or supernatural cause.