Not a 'Bus person myself but it sounds like what PM said which is was quite simply hot brakes.
If you have somehow ended up with hot brakes on taxi out ( e.g. residual heat in brakes from previous landing due to short turnround, then long taxi out, perhaps with a few brake applications, perhaps a hot day, and maybe on a bad day a slightly "dragging" brake) and it's not impossible to get a "Brake Temp" warning or similar after take-off...the solution to which is usually laid down in the appropriate checklist which normally says something like lower the gear again for X minutes.
Worth bearing in mind the brakes can still be absorbing energy and heating up for some time after the final application on taxi out, so it's perfectly possible have no warning on takeoff but for it to appear shortly thereafter in the climb.
Last edited by wiggy; 24th Jul 2017 at 13:57.