Thought I'd share this explanation suggests by a Rabbi centuries ago. First, the prohibition of boiling kid in mothers milk is repeated three times:
Exod 23:18 You shall not offer the blood of My sacrifice with anything leavened; and the fat of My festal offering shall not be left lying until morning. 23:19 The choice first fruits of your soil you shall bring to the house of YHWH your God.You shall not boil a kid in its mother’s milk(
(a doublet at ) Exod 34:25 You shall not offer the blood of My sacrifice with anything leavened; and the sacrifice of the Feast of Passover shall not be left lying until morning. 34:26 The choice first fruits of your soil you shall bring to the house of YHWH your God. You shall not boil a kid in its mother’s milk.
and the Deuternonymists revision at Deut 14:21 You shall not eat anything that has died a natural death; give it to the stranger in your community to eat, or you may sell it to a foreigner. For you are a people consecrated to YHWH your God. You shall not boil a kid in its mother’s milk.
In the Exodus context the emphasis is upon the notion of proper sacrifices, specifically not delaying sacrifice until the full crop has come in while the later Deuteronomist interprets the phrase as being a food prohibition.
R. Joseph ben Isaac Bekhor Shor (12th cent.) Sees the issue as a matter of mistranslation by the Deuteronomist himself.
He writes:
According to the plain meaning, the term “bishul” here means grow or complete, similar to its use [in the verse (Gen. 40:10)]: “its clusters ripened (הבשילו) into grapes.” This is what the verse is saying: do not allow [the kid] to grow up and be weaned from its mothers milk. [In other words, do not] wait until [the kid]’s mother grows it with her milk, rather bring it at the beginning. This fits with the context of the first part of the verse, “the choice first fruits of your soil [you shall bring].”
While, as was mentioned in this thread, there MAY have been some specific ritual in mind, this explanation certainly sounds persuasive.