Maybe.
Yeah it might be a levelling issue (if you own a spirit level then give it a check) but that would be strange. How did you apply your fertilizer? If it was well mixed into the soil then it shouldn't wash to one side that badly. Hardly at all in fact.
Is the better developed side the warmer sunny side (ie: facing South if you're in the Northern hemisphere)? Perhaps there is a temperature differential from one side on the bed to the other?
I also notice that the surviving plants are somewhat more shaded (by the box's
wood). Perhaps what you planted doesn't germinate/survive well in full sun?