Caran, Tony had already bought Liz's share of the pub (she didn't know he was the buyer). Tony told Liz that a big deal of his had fallen through and bad guys were calling in the debt so he had to have the money back that he'd loaned Steve to get him out of the dire straits he was in.
Tony and Tracy knew that the only way Steve could raise that money was to sell his half of the pub - and of course the plan was to then kick Liz and Steve out of the pub and put himself in it.
Liz was not convinced that the new buyer would be content to be a sleeping partner and was worried they'd lose their home so Carla offered to buy the other half - she got a bank loan.
When Tracy found out she was in a rage and set fire to Carla's flat, not knowing that Amie was sleeping there.