A judge has asked federal land managers to explain why they should be allowed to continue capturing more than 2,500 wild horses in northeastern Nevada — a roundup opponents say is illegal and has left 31 mustangs dead in 26 days. Wild Horse Education, a nonprofit seeking to protect the horses, has sued the Bureau of Land Management and is seeking a court order to temporarily halt the roundup halfway between Reno and Salt Lake City. Among other things, it says the agency is violating its own safety standards that prohibit roundups in extreme heat and the use of helicopters to assist in the capture of the animals when foals are present.