There's almost never a good reason for that sort of change.
Moving to a more permissive licence can be tactically useful if the functionality of the software is already widely available in proprietary software *and* wide adoption will help break a form of control that proprietary software companies have on a domain (i.e. via a format or protocol) *and* the copyleft provisions are reducing adoption.
So, for example, releasing Ogg Vorbis and Theora under a permissive licence was a good idea because there were already lots of other more or less equivalent AV formats, and wide adoption would reduce people's demand for .mp3, .aac, .wmv etc., and producers of media players might have been worried about copyleft requirements.
In almost all other cases, moving to a permissive licence will just cause less free software to be written. Anyone know if the developers did this because they believe pump.io falls into the small category above?