I think you're close, that the reason is evolution, but there's a little more to it. They're evolved from cactii, some of which require enormous room for their roots to spread. In this particular species, that's made them territorial, but they needed a way to defend themselves. After millions of years of evolution, they developed a way to detach part of their main surface extrusion from their roots and it travels towards the nearest source of heat (which is why they avoid zombies and skeletons, and also why they come out at night, when the sun can't create heat to obstruct their own tracking). The appearance of eyes and a mouth are also there as a warning, similar to the way other creatures can produce false faces or bright colors. The hissing, however, is not intended as a warning, but is the heat causing the redstone to ignite.
The real physiological explanation for their exploding is uncertain as they are unsafe to approach at any time, but it is suspected that, growing in the ore-rich environment that they do, they unintentionally included some of the volatile redstone so prominent in most areas. A particular breed mutation included what was once a naturally occurring explosive material, and when animals would attempt to graze on them, the increased body heat at the proximity caused a reaction between the redstone and the explosive material. This kept their habitat relatively free of grazing animals and encouraged the mutation to increase in the species. Because of it's powerful defense, the species propagated widely, and has absorbed all sources of the explosive material, which is now produced almost exclusively inside their external shell. Redstone only still exists because it was a deep ore which was revealed later through natural erosion, and many deposits remain deep, beyond the creeper's ability to reach it's roots.
However, at first they remained stationary, and animals grew to understand that, as long as they grazed only on the farthest reaches of their roots, they would be safe. Eventually, the creeper extended its body across all its roots, but this quickly became too much for the roots to maintain, so it instead learned a method of locomotion (much like how fly traps can open and close, or the way ivy climbs up walls). The "legs" as they appear, are actually continuously re-attaching root connections. The roots are doing all the actual movement, pushing the creeper protrusion along the ground by connecting and disconnecting a sequence of roots along the ground towards the goal.
Through all this, the corpses of animals was an unexpected benefit to their growth: it allowed them to spread their roots wider than ever by using these bodies as nutrients, which is why they are so widespread across the land.
The best way to deal with this dangerous plant form is to use some kind of ranged weapon so the plant can't use enough heat to cause the reaction within its body. The goal is to break its outer husk and cause it to lose vital fluids. If it loses enough, the roots will abandon the husk as it becomes too costly for the main creeper plant to maintain it, leaving the explorer to collect the explosive remnants for their own usage.
Sometimes, they eat music discs. No one knows why.
(OK, after writing this, I think I might post a bit of an updated, more well-organized version of this to my blog later. I forgot how fun creating false analyzation for this stuff was.)
