The moment Mufasa stood up, Tanga knew he was in trouble. Standing up himself, Tanga backed off a couple of paces, suddenly frightened once more. Mufasa had caught onto his fear and uncertainty, and knew something was up about his past. "N-n-nothing!" he stammered in a rather bad attempt at casting the subject aside as Mufasa questioned Tanga's reaction to the king's name. "I-I just...I just heard someone at home mention you a few times, that's all!" he explained desperately.
Technically, Tanga was not lying...there were members of his pride that did mention Mufasa from time to time. Tanga just decided to conveniently leave out the fact that those references to him almost always involved plots to kill the king. Mufasa would never let him stay another minute if he ever found that out...he needed to protect that secret at any and all costs.
By this point, tears were forming in Tanga's eyes, the constant references to his violent, and his fear that Mufasa would discover his secret were takign their toll on his emotions. Struggling to hang onto the last pieces of control, Tanga gaze once more fell away from Mufasa's, as he violently wiped away the water in his eyes. "Please, sir..." he pleaded. "Please...I can't talk about this...and I have nowhere else to go..." he was literally pleading by this point. If his family ever found out what he was doing tonight, he was dead for sure. And if Mufasa ever found out about his past, he was also dead. Secrecy was his only chance for survival, in his mind, and he was almost begging Mufasa for it.