This happened because the feed roller(s) dropped into the groove, lowering the cutter head. The rollers are always under pressure so that there is enough friction to keep the wood moving. A filler strip at least near each edge to keep the roller up is one way to prevent it. A sacrificial strip on each side (if there is room) will also work.
With most lunch box planers, the cutter head is on a carriage that also includes the rollers. It pivots in the middle, so when the roller drops, so does the cutter head (about half the distance of the roller). When width allows, a sacrificial piece on each side that is longer both front and back than the distance from the cutters to the roller will prevent snipe on the ends as well is for any dadoes. (basically, the sacrificial pieces have to exceed the length of the snipe occurring).
Be aware that these filler strips may have a tendency to kick up/back when first fed into the rollers, so have a firm grasp on them until the roller starts pulling them in. When the width does not allow, a piece in front and in back tight up against the work will prevent end snipe, but won't stop the drop into a dado.
The filler strip does have to be secured enough that the pressure on the end of it as it goes through the roller does not tilt it up into the cutter head.
