Their song I'mma be was really about being like bees in a hive...think about it...I'mma bee, I'mma bee, I'mma I'mma I'mma bee.
The Peas were on their way back from a tour when their plane tragically crashed in some mountains somewhere. Desperate not to lose them and the huge amounts of revenue they generated, their record label secretly shelled out the money for some Silicon Valley firm to build RidiculouslyHumanRobots of them. Less lifelike robots of them had already been commissioned to be part of the special effects during the Peas' next tour, but now their record label pushed for the Silicon Valley firm that was building them to make them as realistic as possible.
Next, the record label quietly covered up the real Peas' plane crash and gave them quiet private funerals. Meanwhile, the android versions of them were downloaded with the original Peas' voices, speech patterns and mp3's of BEP's older songs, as well as mp3's of the preliminary versions of the songs on the then-in-development The E.N.D. album (since then, due to the robots' retention of the Peas' voices and speech patterns, the record label has been able to "teach" them newer songs by simply downloading the lyrics into their hard drives).
Also, in a fit of rather morbid and creepy inspiration, the record label named their next album The E.N.D. as a secret clue to the fact that the real Peas were gone, but thanks to the wonders of Silicon Valley technology "the Energy Never Dies," now that their voices and likenesses are preserved.
And that, in a nutshell is why ever since The E.N.D., the "Peas" have sounded like robots. Cuz they are.
Either that, OR.....
The paramedics arrived on the scene and found them barely alive. They were rushed to the hospital and due to their immense wealth and the wealth of their record label, it was arranged for them to have an experimental procedure to replace their maimed organs with mechanical and cybernetic replacements. Their voiceboxes were unfortunately among the injured body parts, so they had to get miniature voice-simulating computers installed in their throats to mimic the sounds of their original voices.
This explains not only the robotic voices since The E.N.D. but why they now move more robotically.
Horrified by the fact that huge chunks of their bodies were now machines, and by how their voices now sounded robotic, the Peas started wondering What Measure Is a Non-Human? and panicked that their careers were over, until Will.i.am came up with the idea of hiding their newfound status as cyborgs in plain sight by making "futuristic" their theme from now on. Thus, the concept for The E.N.D. was born (and the title was meant as a way of saying that they ALMOST died, but thanks to the innovations of medical technology, the "Energy Never Dies.")
Since then, their entire masquerade has been based around convincing everyone that their semi-human, semi-robotic mannerisms and computerized voices are all just part of an act. YMMV as to whether they are now more machine than man.