Suckback porosity is due to:
Suck-back porosity often occurs at an occlusoaxial line angle or incisoaxial line angle that is not well rounded. 
A hot spot may retain a localized pool of molten metal after other areas of the casting have solidified. This, in turn, creates a shrinkage void or suck-back porosity. 
This type of porosity defect can be eliminated by flaring the point of sprue attachment and reducing the mold-melt temperature differential—that is, lowering the casting temperature by about 30°C.

Reference: PHILLIPS’ SCIENCE OF DENTAL MATERIALS, 12th ed page no 227
Attached impurities on undersurface of casting
Hot spot
No rapid cooling
Inadequate melting temperature