Why did my lids seal?
Evidently, despite your not doing so intentionally, the evidently the brine was in fact hot enough to heat the air in the head space enough that when it cooled, a partial vacuum was created.  The jars are truly sealed as you noted from your description.

While they are sealed, since you did not process the pickles, they are not certain to be safe for long term storage, even in the refrigerator, and should still be treated as regular refrigerator pickles--eat them within a few weeks.  That probably is not a problem :-)