That (usually called ewe neck) is caused by the horse carrying their head and neck up...the muscles devolpe to help the horse carry their neck in that high of an angle.
Riding the horse in the correct frame will help, but also teaching them to go long and low (have their head line up with their back...sorry really bad at describing things)
"A ewe neck is counter-productive to collection and proper transitions, as the horse only elevates head and doesn’t engage its hind end. The horse's loins and back may become sore"
"The sunken crest often fills if horse ridden correctly into bridle. However, the horse's performance will be limited until the proper muscling is developed."
(two quotes taken from wikipedia)