I'd say trauma but mostly conditioning. When I was young I wasn't afraid of anything and everyone I knew was afraid of something: cockroaches, mice, spiders, heights.. but I wasn't and I thought there must be something wrong. We were living at a rural area, across our house was a rice field, behind was an big empty lot with tall grass fields. I convinced my young self that in these conditions the only logical thing to be afraid of are snakes since they are stealthy, can bite, can't be easily killed and I'm sure there are some crawling around the area. It turned into phobia as I grew up. I witnessed locals pulling out a huge snake from my aunt's bamboo tree so they can kill it and eat it (fairly common from where I'm from) Found myself scared every time I spot movement on grass at my cousin's.

When I got to my pre-teens even a picture can scare me. When I was 12 my friends and I were walking to the beach and I saw what I thought was a local delicacy of isaw(chicken intestines skewered in a stick) and when I picked it up it was actually a small snake. Was so scared I refused to eat isaw for five years even though it was my favorite street food. When I was 14, I was in my room and suddenly heard my brother who was just outside my room saying 'why is that crumpled piece of paper moving??? there's a snake!' I freaked out, shut the door, stood on a chair and looked down on the floor because there was a gap between the door and floor and I knew the snake can slip in. Everyone was panicking outside which freaked me out and then his head popped in and I screamed so loud I scared him away. My Mom killed it and my Dad had to give me a piggyback downstairs because I refused to let my feet touch the floor in case there's more. I haven't had any more experiences since then but I get scared of anything that remotely reminds me of snakes like chickens because their head movement looks like snakes about to attack. I even get scared of my cat when he's on pounce and hunt mode because his movement is so similar to a snake and sometimes the way his fur looks when he moves look like snake skin. I know they are important in our ecosystem but damn I'm too scared.

TLDR; I created my own phobia.