Western Cape SACP secretary Benson Ngqentsu believes Brian Molefe returned as Eskom CEO after he failed to secure a ministerial position in government.
Ngqentsu was speaking to SACP supporters in front of the Cape Town International Convention Centre (CTICC) where the African Utilities Week conference was taking place.
Molefe was expected to open the conference, but unexpectedly cancelled his attendance.
"Brian Molefe wasn't honest when he resigned as Eskom CEO, he left because he thought he was getting a Cabinet position," Ngqentsu said on Tuesday morning.
Molefe was reinstated as Eskom CEO on Monday after a three-month term in the National Assembly.
He resigned as Eskom CEO in November following a report by then-public protector Thuli Madonsela which implicated him in a coal deal with the controversial Gupta family.
Molefe was widely speculated to be Pravin Gordhan's replacement as finance minister, but Ngqentsu said successful lobbying by the SACP ensured Molefe did not receive the position.
"Look what we've done, we must continue fighting against that corruption," Ngqentsu said.
Roughly 50 SACP supporters assembled in front of the building to protest Molefe's reappointment, under the watchful eye of police officials.
Posters calling for an inquiry into PetroSA, Transnet and Eskom were held up by supporters.
"We were expecting Brian, but he's run home crying," one poster read.
Ngqentsu earlier told News24 that Eskom chairperson Ben Ngubane is the "main culprit" responsible for "looting" at the parastatal.
"He is the one that wants to build another Hlaudi [Motsoeneng] at Eskom," Ngqentsu said.
Private security guards were stationed in front of every entrance of the building, while delegates smoked outside and stared at the protesting SACP supporters.
The supporters complied with a police request to move away from the building and into the street as police said the CTICC was "private property".
CTICC spokesperson Susan Davis said they were talking to event organisers to ensure the safety of delegates.
Ngqentsu earlier said the relevant authorities were not available on Monday to gain the necessary permission for the protest.