Ziggs is a full on mage, so you want to get a lot of ap fast. So by the order of the build it will help you get ap fast.

To be a good player and get kills early on the game you want good runes. So with the runes up above with doran's ring you start out with, you will start the game with over 40 ap already.

The reason why you max out bouncing bomb as fast as you can is, because when you start the game with a ton of ap your bomb will deal a lot of damage, especially to those who don't have magic resist. The minefield is not upgraded at any time during the early game is, because you mostly use it to slow down enemies, not to deal damage. So you get just once. Finally your satchel charge is maxed last, because simply you only use it for an escape route or to knock away, or forward enemies, not to deal damage.