There were various reasons why free trade was considered an issue around the 1906 General Election, much of this in my opinion was around the opportunism that was on offer to the Liberal Party. The Conservative Party had been rather factious showing poor leadership under Balfour and had a "3 way split".
Free trade achieved a mandate under the Liberal victory of 1906. There was a use of propaganda with the "Small loaf, big loaf" poster which showed that bread would be expensive under the Tariff Reform. The small loaf represented the fact that food would be affordable (under the Liberals) and the big loaf was there to show that with tariffs, the core necessities would be unaffordable. The issue was that people just wanted cheaper food prices as it was high-priced, this is what brought attention to the "free trade" issue. In the long term, free trade would bring down the price of food as there were fewer restrictions. It was similar to how the way they tried to handle pensions because the rich and lords saw it as robbery from the rich to give to the poor under Asquith.
This was the first time the Liberals were reunited since the Boer war. This aided them in an attempt to reunite there votes aswell. This meant that they were the second biggest party after the conservatives. All they needed to do was show the weaknesses of Balfour. The final nail in the coffin was having a decisive manifesto over free trede and how they refused to do it in case it harmed the working class. It was most important to appease the working class because they were the majority of the vote but they were also the poorest people able to vote with decisiveness.