[SilverBirch]

The markets want to know that the UK can pay off it's debt. The government announced a massive spending package along with massive cuts to tax revenue. As well as this, they did it without the normal independent analysis on the impact, and have signalled they don't think they need to worry about the UK balancing it's budget in the short term. So the markets don't think the UK will be able to pay off its debt because the government announced plans that will cause it not to be able to pay its debt. Simple as that.

Normally left wing governments are punished because they increase spending without increasing tax revenue - causing unsustainable debt. The UK has done the right wing equivalent - they decreased tax revenue without decreasing spending - causing unsustainable debt.