They seem to be especially vicious this year. For many years we used praying mantises to control the yellowjacket and wasp populations around the patio. They would occasionally catch a silver-spotted skipper but didn't usually hang out where the monarchs were. A couple of years ago we ended up with no females or egg cases and didn't re-stock, and now we've got what appear to be whitefaced hornets attacking the butterflies at high speed. They always go for the head, leaving decapitated butterflies in the grass. Don't know about the egg-laying part, but the rest is just as you describe.
I've been keeping my reared butterflies inside longer, making sure they're thoroughly dry and flying strongly before letting them outside.
Certified Monarch Waystation #12
near Philadelphia, Pennsylvania