[en.wikipedia.org] [pbs.org] [burgess-shale.rom.on.ca] - these sources mention several theories on why it happened. These theories are not mutually exclsive, and some proposed causes may produce other proposed causes. From Wikipedia,

• Embryonic-development improvements. Like duplication of Hox genes and associated genes, making more Hox zones, as they might be called. This makes finer-grained specification along the main body axis.
• A mass extinction just before, at the end of the Ediacaran Period.
• Predator-prey arms races.
• Evolution of eyes and hard shells and the like.
• Increase in molecular oxygen, O2.
• Increase in ozone, O3, that increased shielding against solar ultraviolet light.
• Increased dissolved calcium from volcanic eruptions and erosion of mountain ranges.