It's likely because some links aren't pivoting freely, probably because the chain hasn't been lubricated properly. It may not be old or badly worn, but once it won't pivot freely it's had it.
You might get another 1000 kilometers out of it if you spray it with lots of thin oil. This makes a horrible mess but sometimes helps. Be careful that the oil not get on the brake disc. Wipe the excess off a day later, don't use the bike in the meantime, and re-lubricate with good chain lube.
But we'd be tempted to cut our losses and replace it now. A broken chain failing about on a speeding motorcycle can do a lot of damage.