Ahmad ibn Hanbal was a collector of hadith before he was a jurist, and the school that carries his name came about partly because of what happened to him rather than only because of what he taught. From 833 the Abbasid caliphs required judges and scholars to profess that the Quran was created, a doctrine the state had taken over from Mutazili theology. Most complied under pressure. Ibn Hanbal refused, was flogged and imprisoned, and after the policy was abandoned he would not accept the compliance of those who had given way.
That refusal became a reputation, and the reputation became a following. It is an unusual origin. The other Sunni schools formed around a jurist method and the elaboration of it by his students. The Hanbali school formed around a stance on a question of theology and only afterwards acquired the legal apparatus that made it a madhhab at all.
The major work Ibn Hanbal left is the Musnad, a very large collection of hadith arranged by the Companion who transmitted them rather than by legal subject, which is a collector book and not a lawyer one. His own legal positions were preserved by his sons and his students as reported answers to questions rather than as a treatise, and it took later generations to sort them into a system. The origin left a mark on the method. Where an authenticated report and an analogy point in different directions the school follows the report, and it is willing to follow a weak report rather than reason its way past the texts.