General Gigan

Question: In the scene where Tom Cruise and co. are in the tripod's 'basket', a man is sucked up into the tripod and no-one does anything, but as soon as its Cruise's turn everyone grabs him to try and help, why?

Answer: When Tom covered his daughter he noticed a grenade belt dangling from the cage. He grabbed it as he was being pulled into the tripod. When the soldier noticed Tom had the grenades he immediately grabbed Tom to pull him out, and ordered everyone to pull as well.

Answer: You can see someone try to help the first guy, presumably the same soldier as when Tom gets caught.

General Gigan

Answer: Everyone in the basket realised he had a child with him so they decided to save him. Only after they pull him out did they realise the grenade pins were pulled and then the army guy told everyone to brace themselves.