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I guess they're changed routinely after a set number of landings.
Pretty much. And it's actually a bit easier to change them than on most cars, at least ones you and I might have dealt with! It was also probably done since they were waiting for the weather to improve at Singapore. Plane's on the ground, we have an hour more than we need, put the mechanics to work. 
The "gust" was probably a result of the "tropical storm" that was moving toward the airport. Those winds are often the result of rain caused downdrafts in a large thunderstorm. When that downward moving wind reaches the ground, it has to go sideways. If that gust is from behind the plane, the actual airspeed can drop by the speed of the gust. If that speed is 25 knots or more, it can easily approach the safety margins built into the normal air speed for that phase of flight. And it's the AIR speed that keeps the plane in the air, of course. The gust was probably enough of a warning to the crew that the storm was getting too close and too dangerous for safe operations. Besides, most thunderstorms are moving relatively fast; give them even 30 minutes and they'll be 15 miles away! Sounds like a great and correct call by the crew! 
Only a fool argues with 'mother nature!'
Pick your battles and choose only those you have a good chance of living through!