The Labour manifesto in 2019 was fully costed - perhaps relying on some optimistic economic forecasts, but not too concerning to the markets - until they promised to compensate the WASPI women, which would've cost another £58bn, blowing a hole in all their budget forecasts. If they'd gone ahead with a £58bn black hole in their budget then the markets would certainly have reacted badly. That policy undid all the work Macdonald had done to reassure the markets that Corbyn's Labour was not a threat to their investments.
The markets only care about getting a return on their investments. They don't really care what governments do as long as their investments are safe. They get jumpy when left wing governments are elected as left wing governments should be hurting their investments. Allende, for example, nationalised the copper mines without compensation - about as bad for investors as it could get - and the markets reacted punishingly to that.
Markets are usually reassured by right wing governments as they've traditionally been there to protect the market's investments, at whatever human cost. Truss, with a £60bn black hole in her budget, borrowing billions to pay for the energy price cap when UK borrowing is already high following Covid, relying on fantasy growth to be able to cover the cost when only about 3 people in the world think her trickle-down plan will work, has made the markets think their investments aren't as safe as they'd assumed and they reacted. If she and Kwarteng had announced plans to pay for the energy price cap and tax give away which would've worked financially, such as huge cuts to public spending - ending the state pension, funding the NHS through insurance - the markets wouldn't have batted an eyelid.
