Manganese has a half-filled 3d shell (Right? 5 electrons in 5 degenerate orbitals). This imparts an added stabilization to those 5 electrons since the atom as a whole has no angular momentum, because it is spherically symmetric. Similar phenomena take place for technetium and rhenium. Zinc, cadmium and mercury experience a similar but more pronounced stabilization due to a full d shell which experiences not just a stabilization due to spherical symmetry (To put it simply) but also because all of their electrons are paired up. All of the elements just listed also experience an increase in ionization energy since going from a 5 to a 4-electron system would result in a more significant destabilization as compared to going from, say, a 3 to a 2-electron system. Even more evidence for the stabilization in the 5-electron case comes from the fact that chromium, the element that comes before manganese has the electron configuration 4s13d5. Why, when vanadium is 4s23d3? Because the stabilization from putting that next electron into the d shell more than compensates for the fact that you are putting an electron into a higher orbital.

In other words, the 5 electrons of Mn are more okay with being unpaired than, say, Cr or Fe. They do not want to form the metal-metal bonds that other metals form tho make them solid/liquid, as much.

In general, half-filled shells experience a stabilization. The s-orbitals are a trivial case, since we can not really measure the stabilization relative to a quarter filled S orbital, but p-shells also experience a stabilization when you hit 3 p-electrons, and f when you hit 7.