I am not experienced in the topic, and could not find any article related to these kind of reactions, but my guess is the following:
During the nuclear transmutation the radiocarbon will lose an electron (beta particle), so the nitrogen product will have positive charge. The electron shells will collapse a little bit, since the nitrogen has smaller nuclear radius. The part after that depends on which carbon we are talking about in the glucose molecule. I would guess the following rearrangements:
- ${-CH2OH} \rightarrow {-NHOH} + H^+ $
- ${=CHOH} \rightarrow {=NH} + H^+ $
- ${-CHO} \rightarrow {-NO} + H^+ $
It is not easier to break an N-H bond compared to the N-C and N-O bonds according to this table, but I think what really matters here is the stability of the final product, and $H^+$ is way more stable than any $^+O*$, $^+C*$, etc. ion.