Knobbiness and irregular shape—second growth is results when tubers resume growth because of improved environmental conditions after the tuber’s initial expansion under less favourable conditions.
One example is water following drought. Tissue around the apex may resume growth, causing an enlarged end.

Lateral eyes enlarge to produce knobs.
Heat sprouts occur when sprouts develop from potatoes tubers that have suffered from high temperatures and grow either as a sprout or a leafy aboveground stem.

Tuber chaining occurs when a series of secondary tubers develop on a single stolon.
Resumption of potatoes tuber growth following quiescence is often accompanied by carbohydrate translocation from the basal part to another part of the same tuber, leaving a wet, soft mass, jelly end rot.

Also, carbohydrates may move from one tuber to a different tuber more terminally situated on the same stolon. When this or jelly end rot occurs, market quality is greatly reduced.
Some varieties are more prone to injury than others.
More space between plants help avoid excessively large potatoes tubers and to promote uniform stands.

Regulate water supply to provide uniform growing conditions.

Presto Products GKL09515 Geobin Composting System