Spontaneous fermentation after 18 days in secondary

I'm brewing a batch using Nottingham, was getting ready to bottle because it had been in the secondary for 2.5 weeks. I wake up this morning and see that the carboy is full of krausen and that the beer is in a vigorous ferment. The beer had a vigorous fermentation in the primary 3.5 weeks ago.

Has this ever happened to you? What do you think caused it? The temperature is very steady, surely it wasn't a stuck fermentation.

What was your OG after fermentation in the primary? Was it close to your anticipated OG?

Unfortunately if you had a reasonable OG, it could be an infection. Some of the wild yeast strains can break down and eat complex sugars that beer yeast can not.

Hoping that it was an original incomplete fermentation for you.