[stay_upright]

I'd check everything out - you don't want this to happen again!

Some additional things to look for

Cable - if cables wear often they fray as they enter the nipple at the handlebar end - this is difficult to see of you dont check for it specifically - flexing the cable right at the nipple would allow you to see it. I'd remove the cable completely and push/pull the inner cable in and out many times to check for smoothness, maybe sand is in there but only occasionally will jam. I would consider a new cable.

Cable again it could be the outer cable came loose a bit and did not re-seat properly which would result in a partially wide open throttle, exactly what you had, check turning the bars if this affects things as well.

Carb - inspect the slide and needle for score marks if you have any sand has got in there and could have jammed it.

Spring - on the KTM's the carb spring looses tension over time reducing the ability to close the throttle if the cable is a bit rough or some sand/dirt is in the carb. Consider a new one they are cheap.

Throttle - this happened to me the bottom of the throttle casing had broken but as it's bolted to the bars and covered in a rubber boot you could not tell except once when the throttle jammed open. Remove the throttle and inspect both castings.

carb - I'd remove if from the bike inspect and in particular look for and sand or sand sized bits of dirt in the float chanber or elsewhere - and scratch marks around the slide and needle areas.

I would guess it's not the bike running lean (at least as the only cause) since a riders first reaction is to shut the throttle - if the slide closed properly the motor will not have enough air to do anything other than idle - I don't think your slide dropped to the idle position properly, something stopped it.