When it changes profiles it loads the changed settings and some that may not have changed as well - Braille is one of those - so it tries to load the saved Braille driver (for your usual Braille device) but because that takes a few moments to ascertain that there is no driver connected and rollover to the "No Braille" setting, it is more noticeable than when NVDA first starts up.  It might be something we can make more efficient though.