carbing in the keg

I made a pale ale last month - AG and used Munton's Dry yeast. Kept it in primary for four weeks. Went from 1.072 to 1.030. At the three week mark I added an extra packet of Munton's Dry. Nothing much happened.

After a total of four weeks in primary I racked to a keg last week. I decided to keep the keg at room temp to condition further. Each day I open the safety valve - last night when I pulled up on the valve it released quite a bit of pressure.

I opened the lid this morning to take a sample and there was a thick layer of foam on top that looked like head from a freshly poured pint. The OG dropped to 1.024 so apparently some fermentation is still occurring.

Why is there so much pressure building up in the keg? It appears that the beer is carbing on it's own with no co2 or priming sugar added. How does that happen? Does carbonation affect a hydro reading?

My normal process has been thrown off by this beer b/c of the high OG - so things are happening that I am unfamiliar with.

Any help is appreciated.

Your fermentation wasn't complete, so the CO2 generated by the continuing fermentation will absorb into the liquid if not allowed to vent. That is also why your gravity is still dropping. Obviously the act of racking to keg got the yeast started again. Give it another week or so to finish fermenting and then rack into a fresh keg and carbonate.

Yes and yes.

Fermentation creates CO2 if this can not escape through an air lock then it will build pressuer and this will be absorbed by the beer.

The danger is the pressure will impede the completion of the fermentation and you'll end up with a lower ABV and more sugar in the beer than you wanted. I'd fit an airlock and leave be. If you have krausen then you may end up with too much trub.

Edit: Bradsul beat me......what he said.