1. Galilee vs. Judea. To an extent I think Jesus’ disputes owed to a regional interpretation of the Torah. Whether Richard Horsley or Sean Freyne is more right about the northern religion, it’s hard to escape the idea that early Christianity stood for a particular Galilean way of being Israelite. Josephus implies that Galileans adhered to a minimal Torah: circumcision (Vita 112-113,149) and sabbath observance (Vita 159) may reflect basic customs having roots in Galilee prior to Hasmonean takeover (from descendents of northern Israelite peasants left on the land after 722 BCE, as Horsley claims), but not adherence to a highly codified priestly Torah which had developed in southern Judah/Judea.
2. Competing Moral Imperatives. Jesus also entered into Torah debates in the interest of competing moral imperatives. An obvious case is where he appeals to the hunger of David and his men, saying that one imperative can trump another. Human need can override a commandment even if the commandment itself remains intact. Dale Allison notes (in Resurrecting Jesus, chapter 5) that we call this choosing the lesser of two evils. Peasant concerns to take care of their livestock, even on the sabbath, would be another example of pragmatic need trumping a general halakic rule.
3. Rhetoric and Honor. The ancients could be plenty offensive when it suited their needs, especially in order to justify views that were socially advantageous (see Bruce Malina and Richard Rohrbaugh, Social Science Commentary on the Synoptic Gospels passim). Men thrived on the macho game of challenge-and-riposte, and shafting their opponents with rhetorical wit and scripture one-upsmanship. But rhetoric can obscure the real reasons underlying a conflict. Jesus’ prohibition of divorce is an example, targeting a practice that Moses may have allowed but created lots of bad blood and feuding (especially in village settings). But it would have been shameful and weak for Jesus to protest about social problems like this. No one would have taken him seriously. So he “burned” his opponents — overturning Moses with the creation story (Gen. 2:24), cleverly implying that marriage was an unseparable “blood” relationship rather a legal one. In this way he honorably (and scripturally) legitimated what he wanted to accomplish.
4. The Apocalypse. I tend to think that almost everything Jesus said and did (whether it was about Torah or not) points back to the apocalypse in some way. And I’m rather surprised at Ed Sanders and Paula Fredriksen for resisting the conclusion that Jesus at least occasionally rescinded the Torah, while underscoring his millenarian outlook in the same breath. Breaking customs and defying tradition is exactly what apocalyptic movements are known for, from everywhere across the globe. They do this precisely on account of the new age being imminent.
So I doubt there is “a” single reason for Jesus’ alleged conflict with the Torah. Many factors converge to produce a regional, reasonable, macho, and millenarian figure — and above all messianic, who made himself the boss of these issues.