As Grahnlawnoted in December, BEREC and it's office were created by a regulation in November 2009. BEREC replaced the ERG ('European Regulators Group for electronic communications networks and services') and has been seated in Brussels so far.
From what I can see and read, Riga seems to be the only city campaigning for the seat, apparently supported by France, and so the decision might have just been between keeping it in Brussels and moving it to Riga.
But we all know how the EU works and so I'm pretty sure that on Monday we can congratulate Latvia to host yet another EU agency. Celebrations!