Everyone obviously meets the first dragon, Mirmulnir?, outside of White Run at the Tower ruins... but after that is meeting a dragon based on your level? your location? can you trigger a meeting? is it all scripted?

I am only Lev 17 but I've had to fight 4 dragons already, and I haven't even really begun any of the MAIN quests! I'm going mage/enchantment so I decided to take the horse taxi from whiterun to College of Winterhold to do some levelling up. I barely got out of the horse carriage at Winterhold (in fact, I was overhearing a scripted argument between a man and his wife at the gate) when a guard flew by me yelling to watch out ... and the music changed and the dark shadow flew overhead. It was an unnamed dragon, though I didn't catch the kind or color as it was after sunset. He breathed fire. I killed him and no sooner got into the courtyard in the college then ANOTHER dragon attacks! I was in the middle of orientation at the college! Been there for all of 10 minutes! This dragon breathed frost, again didn't see a name, but I dispatched him and went to meet the rest of the trainers/students. Naturally, they're all asleep still so I fast-travelled back to Riverwood to try to level up and fill some soul gems. Before I could get leave Riverwood, just outside the blacksmith shop, a blood dragon swooped in and attacked me. When it was all over, Alvor (the smith) and his wife were DEAD!!! (hope there wasn't a quest involving them!)
I feel like I'm a dragon magnet! 3 dragons in just over one hour???

And I feel like ...what the heck? they can kill NPCs outside of your party? I feel robbed somehow, yet thrilled by the openness of the gameplay and flexibility of the storyline.

Both. Word walls in the open world always have dragons. And after a certain early part of the quest, named dragons can appear in set locations. The rest are random.

But my first two games encountered their second and third dragons in the exact same places. This is probably a fluke, but it has me keeping my eyes open for patterns.

I'm playing my first character still - and I'm only at level 12 (just bought the game in the last week). I have 10 dragon souls saved up and have used an additional 4 souls on assorted shouts. Dragons everywhere, it seems! At least 3 times I've had 2 dragons at once; and in one really cool random encounter I happened upon a dragon and a giant fighting each other. Of course, they both turned their attention to me. One of the most surprising dragon encounters was coming out of the "Diplomatic Immunity" quest with the prisoner I had released. The moment we came out of the ice tunnel a dragon was on the ground waiting for us..

Quest related dragons are scripted, the rest are random, I think you'll find. The reason you find them at word walls is because they're dragon spawn points, as are the burial mounds

Despite my foot travelling and somewhat generous fast travelling over pointless open plains, I barely meet a dragon. Certainly not one in every 2 hours. I am glad, it makes investing Dragon Souls much more considerate action and dragons never outstay their welcome.

But indeed, some are scripted, like the Main Quest and several set pieces and word walls.

The way it works, at least from my observation is that each cell gets a chance at spawning dragons, which increases as you play the game and reverts to default when you kill the dragon/dragons.

That is more or less why if you save before travelling to morthal or something, you will always encounter a dragon in the morthal cell, it might be a different type of dragon and it might do different things, but it will always spawn.

Technically nothing can be random in a world of coding, i imagine that there is more indepth coding we cant see yet, but im pretty sure im right.

I somehow found one word wall in the open world that wasn't guarded by a dragon and I even completed a quest when I learned it.

Anyway, the first dragon in Winterhold attacked me at the same spot, but I didn't get the chance to see the second that you were talking about.
Dragons are random, but I guess there are many spots (villages near towns and other) that the probability of having the same scenario is pretty high.

Edit: I'm also a level 17 mage. I left the main right after I completed the whiterun part, and I also fought 4 dragons, not in the places you fought them, of course.
As you go further in the main quest you get more dragon spawining so if you want more dragons simply continue with the main quest

The first dragon does scale based on your level, it was an ancient dragon for me.

I actually had to force myself several times NOT to reload the save game to the point before the Riverwood attack (for poor Alvor's sake). I just feel so bad that an important NPC would be killed.

At least he didn't leave a widow... she died a few meters from him.