"Jews are not allowed to disturb any man one has converted from Judaism to Christianity, nor may they assail him with any outrage. Such behavior will punished according to the nature of the act . [CT 16.8.5]"
This same law is repeated elsewhere:
"...if any Jew unlocks he door of eternal life and delivers himself to our holy worship by choosing to be a Christian, he must not suffer any harassment or molestation from the Jews.
If any Jew dares assail someone who has converted from Judaism to Christianity, that contriver must be subjected to avenging punishments in proportion to the nature of the crime ..." [CS 4]
Given the sort of pressure minority Jewish communities have always been under, it shouldn't be surprising if Jews react badly when some of their community are converted to another religion — especially Christianity, which at this time probably still looks more like a heretical offshoot of Judaism than an entirely separate religion.
Harassment of Jewish converts to Christianity must be more than a minor nuisance, though, for Emperor Constantine the Great to issue a law to specifically deal with it. Aside from protecting Christians and putting Jews at a disadvantage, this new law helps send the message that Christianity is the favored religion in the Roman Empire.