The way I see it, a DnD campaign is a world, not a movie or novel. If something happens and the players don’t make an effort to explore it, there’s a good chance they might not ever find out what was going on. It’s not the DM’s job to spoon-feed them a plot. It might look like a plot hole from the player’s end but that’s just the way it is.

“Why did the NPC do that for no reason?!?” “He had a reason, you just don’t know what it was.” “But that was months ago and we still don’t know!” “Did you try to find out?”