Google doesn't typically sue others over patents, in fact, it's been a strong opponent of most patents and patent lawsuits.
The move comes after BT sued Google back in 2011. Now, Google is using four of its patents, some of which originate from IBM, to sue the telecom giant over some of its services and products, a teleconferencing product in particular.
Google has confirmed the lawsuit and said it was a "last resort." It believes it is the only option when dealing with BT's "meritless" claims.
BT hasn't even been formally served in the UK yet and it's going to be a long sludge through the courtrooms, so expect this to drag on for a while, unless the two companies work something out.