Why did this plane fly in a zigzag pattern? I was tracking flight NKS739 from LAX to SEA this morning and saw that it made some some odd turns. If you look at the track from FlightRadar24.com you can see that it was heading toward SEA then it took a large turn to the right then to the left then returned to its original course.



This does not look like the usual technical glitch from Flightradar24, and FlightAware shows the same track. There was no weather anywhere in the area. There were many aircraft in the area at the time and none of them made any unusual turns. There are no MOA's in the area (most of it is over a national forest) and none of the other aircraft seem to be avoiding anything in the area. I tried to see if LiveATC.net had a recording but that's Seattle center's area and they don't appear to have a feed for ZSE.

I can't come up with any reason for such an odd deviation. The aircraft arrived in SEA and there didn't seem to be any problem. It immediately turned around and made another flight which was uneventful. 

Is there anything that might explain this deviation? Is it just a glitch?
Alrighty then, that took some detective work.
Here's another flight around the same time, SkyWest 4458, which arrived 2 minutes later. Notice it's one deviation, compared to two deviations in your example.

Weather seemed fine, but KSEA has TBFM (Time Based Flow Management) since Aug 2013. If the ATC asks a plane to delay its arrival time, the way it's done is to deviate from the route, then get back on it again—that's if slowing down alone won't meet the time target.
Pilots then use the RTA (Required Time of Arrival) function of the FMC. Initially the function was rarely used, but now with NextGen in the U.S., SESAR in Europe, and similar systems elsewhere, it is frequently used.
It can be arranged way ahead of the airport, i.e., it can be ordered by the approach controller, yet the center controller of another ARTCC delivers the message.
It looks exactly like that. Deviate, if not enough, deviate the other way.
Behind the Scenes
The system is called ERAM (En-Route Automation Modernization).
ERAM detects future conflicts, and based on that, traffic is asked to speed up, slow down, or deviate to slow down further. It reduces/eliminates hold time.
Europe—the first to use it in 2012—refers to it as initial 4D trajectory management. Planes arrive at waypoints at agreed times.
For whatever reason—most probably sudden traffic influx—they were asked to slow down.

The difference between this, and the regular vectoring/doglegging in the terminal area, is the way it is predicted, coordinated, and then relayed to airplanes far from the airport.
Below you can see as the jet turned, it slowed down as well. The initial upward spike is just the tailwind when the plane turned.

Source: flightradar24.com