A fine watercolour branch became broad midway. What is a likely cause?

Correct answer: Brush pressure increased midway

The governing concept is pressure control in a brushstroke. A fine branch is made with the pointed tip and light, even pressure. If pressure increases midway, the bristles spread and release a wider mark, producing the observed change in width. Excess water or paint can cause general spreading, so it is a possible secondary problem, but a local widening that occurs exactly midway most directly indicates a pressure change. A rough surface may interrupt a line, and a dry layer cannot explain this fresh broadening.