I often feel this way too, and wetld spend a portion of every day thinking about it. I did read something that described it as being an Anxiety Disorder and that it can cause people to be very stressed, but for me, strangely, it doesn't seem to stress me out.
My biggest fear is car accidents, to the point where if we are in the car (DP driving-and he's a very careful driver which makes it weirder) on a country road I literally imagine a head-on collision with every car that passes. I think of what position I will get into to try and survive, I try and make him drive really close to the side of the road and always tell him if another car is too close just drive into the ditch to be safe. my talking like this freaks him out when driving (understandably). He works about a 10 minute drive away but as he starts/finishes at 5am/midnight (shift work) I am convinced a drunk driver is going to slam into him. I always make him text as soon as he arrives somewhere and I get into a state if he doesn't and I can't get hold of him, and obsessively check online for news of a fatal accident.
I got really upset when he decided to buy a brand new small car rather than an older big (therefore IMO safe) car. He said the new car was safer.
It's not just about DP either, I'm the same about my parents and myself. I once worked for a year in another part of the UK which meant a flight commuting every Monday and Friday. I'm not an anxious flyer at all, but I would calmly spend the whole flight looking out the window to see where the pilot should crash-land if he had to, eg I'd think "ok, there's a motorway, he could try and land there, the cars should see him coming and brake, and I think the wings are high enough that they won't be ripped off by the central reservation" or "we could land in the sea there, I could swim to the shore from that distance, but would I get my suitcase back?" but again as I say these thoughts don't panic me per say, I feel very calm thinking about it and I don't know why.
DP and I were talking about babies the other night and I said "there are too things you need to consider, one are you ready to have a little person completely dependent on you everyday for X number of years. And two, are you prepared that something might go wrong, eg I might die in childbirth etc and you'll need to do it completely on your own?" He thought that was a really weird thing to say. 
I haven't had any trauma that I know of that makes me feel like this. My parents both had cancer when I was a teenager (not at the same time) but they both survived so I'm one of the lucky ones.
I watched one of those Motorway Police programmes about 4 years ago and a lorry driver was looking at his phone and slammed into the back of a queue of stationary traffic, crushing the car of a young girl (who died
) and even crushing those in the car in front of her who just about survived. I think about that accident every single day, how her family and boyfriend were just going about their life on day then bam, everything changes. This did serve to validate my thoughts that it can happen to anyone.
Is it unusual to think such awful things but yet not be stressed out by them?