What determines where the next blocks spawn?

In Fractal, as you clear blocks, new blocks spawn across the board;

Is there any reasoning behind where the new blocks spawn? Is it based on the blocks you've just cleared (because in my mind this doesn't appear to be the case). Is this completely random?

What determines where the next blocks spawn?

Campaign mode is random:

[...] because your starting field is randomized in campaign mode, you can get everything from a 60 points head start to an unsolvable mess.

http://steamcommunity.com/profiles/76561197970582726/recommended/61310/

Puzzle mode is not random:

In the campaign you get to play through 30 levels of randomly generated fields with given conditions, like limited turns or several colours. Goal is to get enough tiles cleared in a given amount of turns to proceed.

In the puzzle mode you have a limited amount of turns, where you get to fulfill puzzles or given conditions in order to unlock harder and longer ones. The fields are not randomly generated.

http://steamcommunity.com/id/Foxy_Opa/recommended/61310/