I read in another post somewhere on here where this guy had to do some of the MQ before having the game "unlock" dragons so they randomly appear so to speak, or something kicks in where it makes them be anywhere. Is this true or can I hold off from doing any of the MQ and still go dragon hunting?

At least unlock your first dragon shout with the Grey Beards before dragon hunting. I think that increases their amount of spawns.

I started seeing them immediately after taking out the first one, which is part of the main quest. They appear randomly in the sky. Sometimes they land. Sometimes they fly off and disappear. Sometimes they blast me and or nearby NPCs with breath weapons while hovering in mid air.

Dragons can be jerks.

I never saw a single random dragon until a particular point in the mq immediately after I killed the second dragon (a kind of set piece). I am sure it was here because in a debrief conversation the NPC warned me to "watch the skies" before they split. After that dragons were popping up every 20 minutes (Including very inconveniently when I was just about to climb onto a wagon in my 'party clothes' and had no robes, potions or anything, ha)

It might be keyed to killing the first dragon, plus being some minimum level or something. I was lvl 6 I think when I killed the second one.