No, they don't usually grow forever, that's true. But floods happen, and that's bad. Last time methane was this high it was millions of years ago, and Earth had no ice caps. We have already flooded the atmosphere with methane, and the consequences of this will take decades, if not centuries, to play out. Contrary to our current instant-gratification dopamine loops of today, the lag between cause and effect isn't two damn seconds or even one frickin year. So stay tuned.
Was it all from agriculture? We don't know. There's plenty of methane coming from fossil fuel production. Just go read the Wikipedia article I linked. We do know that we are getting close to setting off some very bad feedback loops, as arctic permafrost is starting to thaw, and it's going to produce gobs of methane.