During casting procedures, creation of a hot spot indicates :
Angulation of the sprue former

It should not be attached at right angle to a broad surface because the molten metal may impinge the mold surface parts to produce a so-called 'Hot spot' producing suck back porosity.
An angle of 45 degrees is usually preferred. The sprue former should be directed away from any thin or delicate parts of the pattern, since molten metal may abrade or fracture investment in this area and result in casting failure.
When molten metal strikes against the mold wall at 90-degree angle and if the mold metal temperature differential is more, it creates a "hot spot", the molten metal in this area will solidify last, leading to localizing shrinkage porosity, if there is no continuous supply of molten metal.

Phillip’s 12th edition page 214
The presence of reservoir resulting in delayed solidification of the metal.
Sprue former is attached at right angle.
The length of sprue is too long resulting in the ready flow of molten metal.
Use of too high temperature resulting in localized disintegration of metal.