The presence of Irregular forms
One possible reason is that Children set up rules for their acquisition of a language. This hypothesis is backed up by the Dual mechanism model whereby it suggests that there are 2 mechanisms, rule and memory. So for example the rule is that ‘-ed’ denotes past tense marker. Memory would be the correct forms at work. In cases where recall from memory instantly retrieves the correct form the rule would not be executed. However if the in cases where retrieval does not happen fast enough, the rule would be executed. And in the cases of children because their encounter is still limited. In addition, they have relatively poor memory and/or processing abilities hence when exceptions appear, it would make them more likely to over-regularize inconsistent input as well.
Figuring out the grammatical rules
Children produce terms such as ‘bringed’ and ‘goed’. However these forms do not appear in adult speech. Hence it is sure that they have not learned this words from adults therefore it is not through imitation that they acquire these words. A possible explaination for that would be that children are figuring out grammatical rules themselves and would later modify their rules to accomodate the exceptions as well.
Yet to fully acquire verb meanings
It is also possible to hypothesize that children have yet to fully acquire the meaning of the verbs due to their limited encounters. Therefore they are unable to assimilate the verbs into its narrower semantic classes. An example would be how children classifies the word ‘told’ and ‘whispered’ as a mode of communication, hence overregularizing the word to form sentences like the one in the example below:
“Susan whispered me a story” instead of “Susan told me a story”.