Why did my lids seal? This has never happened to me before!  Last night I made Refrigerator Pickles using cucumbers and an ordinary vinegar, water, sugar brine.  My intention was to make a couple jars for co-workers who would use them within a few weeks.

I sterilized my jars, lids, and rings but didn't keep the jars hot.  The brine simmered but never boiled and was allowed to cool down quite a bit (not completely cool to room temperature and still warm). I filled the jar with my vegetables and added the warmish brine.  Then I added hot lids (I was simultaneously actually canning apple butter so they were simmering), sealed with rings, and set aside to cool.  Within just a few minutes I heard the lovely popping sound all canners long for!  

I don't trust it for long term storage since I didn't go through the actual entire canning process.  Once the jars cooled to room temperature, I put them in the refrigerator; this morning, I tested the lids and they are in fact sealed.  Are they safe for long-term refrigerator storage?  I've had trouble in my early attempts at getting a jar to seal but  never had a batch seal on me unintended!  :)
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 :-)