Why is this happening?

notice how 3d5 4s1 corresponds to a half filled d shell with 5 electrons. This half filled configuration is more stable and preferred over 3d4 4s2 because in the former there are 5 orbitals with five electrons, this corresponds to the maximum number of parallel spins which gives a more stable atom than if there where 4 lone parallel spinning electrons as is in 3d4 4s2. so an electron from the 4s orbital (which is lower in E than 3d) is promoted to a 3d orbital to complete the 5 parallel set.