Arcing is an electrical discharge. This happens occasionally during warmup. To prevent arcing during a scan, be sure to warm up the tube before you start.
When arcing becomes frequent, it is likely arcing off of something - either a scratch or particle in the tube. A brown film can develop over time that can also cause arcing. This can periodically (couple times a year) be cleaned out with alcohol and chem wipes.
Following cleaning the tube with alcohol and chem wipes, there may be a lot of arcing during the next warm up, as it burns away particles left in the tube. Venting the tube and warming up again may help.
If arcing continues to happen, particularly consistently at the same kV, check for scratches.