What causes sub-second mains voltage outages?
If someone turns ON a large load (my table saw does this), it will cause the mains voltage to sag for a second or three and then it recovers while the load is still on.  There are several reasons for this.  Here they are, in rough order of decreasing probability:


Many devices consume the highest power immediately after power-on and then quickly settle down to something normal.  This is sometimes called "inrush current", but in a table saw this is "startup current".  If the power distribution wires have lots of resistance due to length or corrosion then this effect will be worse.
The power distribution wires also have inductance.  If the inductance is large and someone turns on a large load (without inrush/startup current) then you could see a similar effect where the mains voltage dips for a few moments.  
Someone turns on a large load and the voltage dips, the power company notices this and adjusts it's output voltage to compensate.  This one is a weird one because depending on where you are relative to the power company and the load you might actually see the lights get brighter rather than dimmer!  
There is a "disturbance" in the power company and they have to switch from one power source to another.  Normally this isn't a problem, but mistakes do happen.