I have seen in others games that random theives will sometimes attack towns. They are dressed in the thieves guild armor and are seperate occurences from the random encounter along the roads where a thief will demand your valuables. I have never gotten these in my game, and am not too keen on having them disturb the quiet populations of all my favorite little towns dotting the landscape.

Are these thieves triggered if you join the thieves guild? What starts them attacking?

this happens occasionally only in riften (i havent seen them in any other town), and they seem to become more common when you have the thieves guild restored.

I have seen it happen in other towns besides Riften.

Once the Thieves Guild is restored it's nightly in Riften as best I can tell. The highway bandits are random, but not common. If you are TG then there's sometimes an option to say "I'm Guild" and then be let go. But this isn't always present - it may need an article of TG clothing on? I've only seen one attack elsewhere, in Whiterun. The bodies are great for free loot...

Also if you are wearing the nightingale armor the road side thieves actually give you money.

I've had a few come up to me and give me 500 gold saying they had a job go well and this is my share.

Does this only happen if you're wearing Nightingale armor? I had it happen a few times when wearing the armor, but will Guild Master's armor also work?

I dont really know if it happens if you wear other armor cos thats the only armor i use on that character (i'm a thief) but iirc the thief come up to you and says "hail nightingale" so maybe you could wear other armor and it still happens as long as you are a nightingale.

Do these encounters begin when you join the thieves guild, then, and just intensify if you complete the quest?