The Yavetz states that Rav Nachman Bar Yitzchak was unable to control himself from cursing Rav Ada due to his embarrassment. Once he did so, the curse took effect, as we see that curses on people who have done something (even inadvertently) to earn that curse have great power. This is all the more so when the curse is from a Tzadik. Tosfos (DH "Ana Anishtei") indeed says that these Amoraim were blaming themselves in order to do Teshuvah for having led to the punishment of Rav Ada. This was due to the Gemara that anyone who his friend is punished due to him is not let within the "Mechitzah" of Hash-m (Shabbos 149b).

All the best,

Yaakov Montrose