‘And they told Sisera that Barak the son of Abinoam had gone up to Mount Tabor.'
“They” is general. There were many Canaanites who would not look happily on an Israelite rebellion. It would suit them for Sisera to learn of it immediately. ‘Barak is out to cause trouble and has gathered some fighting men on Mount Tabor', they would tell Sisera.
Sisera would know that the force could not be too large from the fact that they were on Mount Tabor. He probably never dreamed that they actually expected to fight his chariot force, but appreciated that when Israelites banded together it was Canaanites who would suffer. And he did not want armed bands on his territory. It is possible, however, that he also received information that the call had gone out to other tribes. Thus he would then know that the threat might soon be a major one, and had to be dealt with at once. The expectation of others joining them would explain why they were waiting in a place where his chariots could not touch them. The only thing to do was stamp out he rebellion immediately. The last thing he considered was that they were there as a provocation to him.