You are processing too many negatives at once, particularly for such a short development time, and you may not have enough solution in the trays. I'd consider handling 15 negs at a time in open trays to be a fairly advanced technique--as in, many very experienced darkroom workers are impressed by stories that Jock Sturges can process 16 8x10" sheets at a time in open trays. Maybe more people did this in the days when there was only sheet film, but unless you're processing a high volume of sheet film every day, work up from, say, 4 sheets at a time, and when you've got it down, add one or two more sheets.
What do you mean "the tray is rocked continually"? It sounds like you are attempting to put the negs loose side by side in a large tray and rocking the tray. This is definitely going to produces overlapping negs and development problems, exactly like the ones shown in the two images you've posted.
The conventional way to process in open trays, unless you're using a "slosher" or some similar frame for holding the sheets separately side by side, is to stack the film and shuffle through the stack to agitate, being careful not to damage the sheets as they are shuffled. There are a variety of twists on this method--face up or face down, angling the tray, how much solution to use, how to place the sheet from the bottom of the stack on the top, etc.--but it doesn't involve rocking the tray. There's a good description with photos in Ansel Adams' book, _The Negative_.