Density varies depending mainly upon i)atomic mass, ii) atomic radius and iii) packing type.
i) Atomic masses increase across the period and this tend to increase densities across the period
ii) Across the first transition series (3d series)-as in any other d-series- electrons are entering the same set of orbitals and so atomic sizes tend to decrease due to enhanced nuclear electron attractions. Beyond manganese electrons start pairing up in d- orbitals which has a counter effect of electron-electron repulsion that tend to expand the electron cloud as a whole. So we atomic sizes decrease up to the middle and then remain nearly same or register small increases as we see: Sc(162pm), Ti(147pm), V(134pm), Cr(128pm), Mn(127pm), Fe(126pm), Co(125pm), Ni(124pm), Cu(128pm), Zn(134pm)
iii) Packings: Sc(HCP), Ti(HCP), V(BCC), Cr(BCC), Mn(BCC),Fe(BCC),Co(HCP),Ni(FCC),Cu(FCC)),Zn(HCP); here BCC has 68% packing efficiency where as HCP as well as FCC(CCP) packings have 74%. So based on packing efficiencies an drop of densities are expected for V,Cr, Mn and Fe
Corresponding Densities:Sc(2.98), Ti(4.5), V(6.0), Cr(7.2), Mn(7.2), Fe(7.87),Co(8.9), Ni(8.9), Cu(8.96), Zn(7.14)
P.S. For the first transition series there are no inner d-electrons or f-electrons to influence the atomic sizes and so densities.
Just qualitative evaluation alone cannot help to predict an order of densities because the factors are not all acting in the same directions.Probably Mother Nature has no intention that things always must be in regular order so that man can study them easily ;)