What can be the cause of a sudden explosion in the loss when training a CNN (Deeplab)
One possible cause is a high learning rate. High values of this hyperparameter usually cause updates that are too drastic, and therefore divergence from the optimum.

Please keep in mind this is only a suggestion, your problem might be due to completely different reasons. Try different learning rates and schedules, in order to understand if that's the case. You can check this very good post on how to set the learning rate.

Also, consider using early stopping to save some training time.