I read that swinging temperatures from warm to cold can cause that. Having warm(75*f+) during the day and then cooling down to less than 60*f at night is what the information said would cause yellowing at the tops of the plants.
I have some plants that look very similar to yours and I have had some issues with them being in the low to mid 70*f and then during regular night when it gets cold here, their temps fall into the low to mid 50s. I have added extra lighting and closed up their growing area against the cold and now their tems stay in the mid 70s for the past week. Since they are still in veg, I am keeping the lights on constantly which should maintain the temps. I should see some changes in another week if the temps swings are the problem.
Are you maintaining steady temps within your grow area?