Is your map suddenly and inexplicably devoid of any random events markers and you don't know why? It'simple. You're too rich.

Yeah, you read that right. Random events in GTA online are a great way to make a quick buck, though you won't be getting filthy rich off of them any time quick. With the various updates over the months continuously adding more and more content to GTA Online, and much of it expensive, players are in a constant drive to earn as much GTA $ as quickly as possible.
Especially with the kind of content floating around like a GTA $ 10 million golden jet, and the ridiculously expensive Lowriders, not to mention the upcoming yacht which has been rumored and leaked. Hoarding cash is something all players want.
Heists may be the best method for this, but random events are enough to cover every-day expenses. However, with everyone collecting money in a frenzy, they might forget to deposit and invest their cash, and they'll soon realize a stunning lack of random events.
Basically, GTA Online detects the amount of money you have (also the basis of some anti-cheat protocols), and if you're running around packed, it won't be spawning low-yield random events. However, if you invest all your funds into stocks, the game will resume throwing its random events at you to help you get earning.