Yes, they do happen, but for sound business reasons and often against strong opposition.
The issue with the NHS is that it doesn't make sense from a business point of view.
The Conservative Party promised a 24/7 NHS in its manifesto and quoted some statistics which have now been discredited to support its ambition.
I don't know exactly what proportion of the NHS budget is spent on staffing, but staffing costs are the biggest cost to the NHS, as health care is labour intensive.
In order to provide the same service on two extra days, the staffing bill has to increase - not quite by 40%, because some staff already work weekends, but by a huge amount. No extra funding is being provided for weekend staffing, so something has to give. There's no money to recruit extra staff, so staff will need to be taken off weekday rotas, which will mean that the service provided during the week will decline.
The government seems to be doing everything on the cheap. Thousands of people are going to find themselves having to work at weekends for no good reason, which will disrupt family life and increase childcare costs, etc. Without recruiting extra staff, the services provided during the week will suffer. Staff are going to end up doing unpaid overtime, which could be life-threatening.
Junior doctors are a soft target, because they are employed on short term contracts in the early years, which need to be renewed on an annual basis. Therefore, the government avoids the issue of having to change existing contracts, but it might not find it so easy to impose contract changes on other NHS staff