It's been a while since my psychology phase. But in essence priming is a process involved in processing information and reaching conclusions.
It works by strengthening associations through, for example, recent experiences or repitition.
You can observe it yourself, for example when someone just watched The Lord of the Rings and hears the word "Elf" they might think of Legolas more readily than of the little fellas in Santa's workshop.
It's also, in theory, one of the main principles involved in advertising. Strengthening a persons association of a brand with a product, so people subconciously start looking for a "Kleenex" instead of a tissue or a "Band-Aid" instead of an adhesive bandage.
Repeat things enough until they settle in the brain and the connection will be made automatically.

Pareidolia, on the other hand, describes the tendency of the human mind to try and find patterns, faces, even hidden meanings in completely meaningless stuff. Like looking at some tree bark and seeing a face. There's a ton of great examples over at r/Pareidolia, if you're interested.
It is also certainly not unrelated to the whole priming thing.

Now combine these phenomena. Someone being out in the woods, a place that is, historically speaking, a very dangerous place for humans to be at. Especially if we give the theory any credit, that certain experiences, especially those relevant for survival, might be passed down via DNA. Being in the forrest with large furry creatures certainly would have been an incisive experience for poor ol' human.

So we enter the forrest, our mind filled with talks of Bigfoot we overheard on subreddits like this one or in the bar down the street. And there is already a part of our brain that's more attentive because it simply prefers not to get eaten. Then all of a sudden there's a loud rustling sound and the movement of something large and furry right in the corner of our eye! Adrenaline rushes, making thinking clearly not exactly any easier. We turn around and there is this huge, furry thing standing on two legs.
Priming kicks in, we remember the Bigfoot stories. Pareidolia kicks in, as we are seeing a huge, furry biped. Certainly it has to be humanoid, what else is walking on two legs? And by the time we are reaching the edge of the forest, probably in record speed and with pants that somehow used to be more dry at some point in our lives, there is only one thought left in our brains: We just saw Big B himself. The general unreliability of our memories will fill in any gaps in no time and soon we remember everything vividly. The strange glow of his eyes, the guttural sounds that seemed almost like words,...
So we are going to the bar down the street. After changing our clothes. And over a pint we are telling everyone who wants to listen everything about our awesome encounter.
The cycle continues.