For me, after resuming from standby in the morning this happens most often. The comic strip starts to show it is busy, obviously trying to update to the new comics. However, it just gets stuck in that update phase, causing plasmashell to go mental with CPU usage. It never resolves from this, and it never fetches new comics. Switching the comic or going back one strip can solve the issue, since that cancels the retrieval. There is sadly no "reload strip now" functionality for manually running the reload. It feels like a loop is getting stuck with endless errors, immediately retrying something, without sleeps, and no abort counter. I have no idea how to log this.
Although to be honest, plasmashell tends to go nuts with cpu in many cases, so in the end maybe it is finally time to overwork how it handles widget authority. It's nuts one malfunctioning widget or an animation with too many redraws can cause it to go full cpu usage.
*** This bug has been marked as a duplicate of bug 363292 ***