The last 2 times I have walked over the bridge to TJ - arriving at 4pm and leaving at 6:30 to 7pm  

Both times I experienced crossing the border in less than 10 minutes both times were on a Thursday.  

I have never seen the lines this short.  I am happy about this but wondering what have been the changes.  

Last year - most of my friends did not want to come due to excessive line wait - at times over 1 hour to cross at 7pm.  

Any insights on why the short lines

Well the lines may be short because of the day of week and especially weather. It seems the hotter it is the lines are like crazy long. Plus if you went around easter all the spring breakers and vacationers were there to make the line long too. I also go on Thurs days and even Friday nights and it is easy coming back.  
Another factor of short lines is that there is the walking cross at Otay that is easy to cross too. There is less traffic there and people know it so the lines pressure is relieved a lot on san ysidro  too. Out of the last 10 visits I. The past 3 months I have been in a line over and hr and that was crossing at a Saturday  morning

Lines have been quicker since New Years.   Even for vehicle traffic.

True,

Not sure why, but thus far 2015 is better than 2014 in terms of border crossing time.