Recent research has revealed several languages (e.g. Chintang, Rarámuri, Tagalog, Murrinhpatha) that challenge the general expectation of strict sequential ordering in morphological structure. However, it has remained unclear whether these languages exhibit random placement of affixes or whether there are some underlying probabilistic principles that predict their placement. Here we address this question for verbal agreement markers and hypothesize a probabilistic universal of category clustering, with two effects: (i) markers in paradigmatic opposition tend to be placed in the same morphological position (‘paradigmatic alignment’; Crysmann & Bonami 2016); (ii) morphological positions tend to be categorically uniform (‘featural coherence’; Stump 2001). We first show in a corpus study that category clustering drives the distribution of agreement prefixes in speakers’ production of Chintang, a language where prefix placement is not constrained by any categorical rules of sequential ordering. We then show in a typological study that the same principle also shapes the evolution of morphological structure: although exceptions are attested, paradigms are much more likely to obey rather than to violate the principle. Category clustering is therefore a good candidate for a universal force shaping the structure and use of language, potentially due to benefits in processing and learning.