Commonly occurring linguistic forms, including allomorphs, tend to be learned (listed in speakers' mental lexicons) even if they are formed according to the pattern of a linguistic rule. They thus have dual motivation: the motivation given by the rule, and the suppletive motivation of their having been learned. This accounts for the otherwise inexplicable persistence of rule-governed allomorphy when the conditioning environment is destroyed through diachronic change, producing apparent positive exceptions to the rule.
"Rule-governed allomorphy can be suppletive also,"
Work Papers of the Summer Institute of Linguistics, University of North Dakota Session: Vol. 41, Article 7.
Available at: https://commons.und.edu/sil-work-papers/vol41/iss1/7