Pashu never realized his responsibilities towards Malti when he was first married. He realized the responsibilities of a husband after watching Tej. And Tej also never dared to support Toasty openly because Pashu used to throw his failed marriage in everyone's face. He used to make Tej feel guilty about showing the least bit of support to Toasty and also used to instigate Pitaji and Ved against Toasty all the time. Taking out his bitterness of failed marriage by not letting his brother have a happy married life.
(Pashu, incidentally, has always blamed others for his failures. Pitaji for not letting him study, the family for his failed marriage, the brothers for not supporting him when he failed to manage the shop and now Tej for making him look like an idiot.)
When Pashu got Malti back in his life, he wanted her to have a higher place than Toasty. But Malti cannot achieve what Toasty did, because Toasty is so selfless, while Malti like Pashu is timid and a little bit selfish.
Once Pashu started supporting Malti, Tej felt that he could also stand up for Toasty. The first time Tej opposed Pashu was when Pashu said "good Toasty is not here, so people will learn to appreciate Malti". From the first dissent it was a natural progression towards standing up for himself.
Even though Pitaji has given Pashu the responsibility of the shop he lacks the maturity to act as a mentor to his brothers or the street-smartness to cope with the business. Therefore he is blustering his way and making Tej the guilty party and not letting Tej speak anything in his defense.