Likely under oxygenation of the wort upon pitching, or under pitched the yeast.
at 1.050 it still had a LONG way to go in fermentation, when you bottled, assuming you used a bottling bucket, you added some extra o2 to the mixture and essentially gave your wort a bit of a jump start. I assume you used wlp570. If you were pitching purepitch packs, you'd need at least 3 FRESH packs of yeast. or one pack of yeast on a 3L starter with stir plate to get the proper pitching rate. (Check the dates! sometimes you'll get 6 month old yeast, at that point its basically garbage)
Additionally 15C is far too cold for that yeast to effectively do its job, per White Labs WLP570 ferments best between 68-75F (20-24C)
the beersmith yeast pitch rate calculator would show that you'd need around 350B cells for a beer of that gravity.
I'm gonna guess this is one of your first higher gravity brews? You'll find once you get above 1.060 you really need to oxygenate, pitch lots of yeast, and make sure you're hitting the temps. If it seems stalled out, it probably is!
I did the same thing on my first high gravity brew, except thankfully i was kegging it! brewed to 1.080, thought that it just finished up at 1.035 instead of the predicted 1.015 and kegged. made a helluva mess once it finally chugged its way to the finish line!