Why do some commercial flights start off unpressurized?
The 737 like most modern airliners (notable exception, the 787) is pressurized by bleed air, from either the engines or the APU. Drawing bleed air from the engines reduces the thrust available for takeoff.

From most common to least common, takeoff configurations are:


Engine bleeds on (supplying bleed air to the pneumatic system), APU off. This is a pressurized takeoff &amp; generally the most common scenario.
Engine bleeds off, APU bleed supplying air to the pneumatic system. This also results in a pressurized takeoff, with more engine thrust available. Used when max thrust is desired, typically due to heavy weights, short and/or contaminated runways, close-in obstacles requiring steep climb gradients, and similar circumstances.


In the case that the APU is inoperative but taking off with the engine bleeds off is still required, you get:


Engine bleeds off, APU bleed off. This results in the unpressurized takeoff referenced in the question.  At a suitable altitude (typically 2000 to 3000 feet into the climb, a minute or two after takeoff), the engine bleeds are turned back on &amp; the aircraft pressurizes normally.


This is uncommon, but still considered a normal procedure (not an abnormal one out of the QRH).

Edit:

Link to Pneumatic System description for the 737

Link to PDF discussing pneumatics &amp; pressurization on the 737