Here in the US, where I am spending a few weeks, there is a new game emerging. It is called Blame The Donald.

The rules are simple: something goes wrong and everyone blames Donald Trump, the frontrunner for the Republican Party presidential nomination.

Often, everything actually is Trump’s fault, according to the Democrats, but I can’t put the blame onto his bouffanted head for the recently plummeting share prices of the US’ largest media companies.

Instead, the first reason is advertising income, which has been rolling into the digital domain for several years. Now that trickle is becoming a surge and it’s hurting even the biggest American TV giants.

Last week’s numbers were jaw-dropping: CBS shares fell 6%, Disney tumbled 9%, 21st Century Fox and Discovery Communications dropped 11% each, and poor old Viacom crashed by an astonishing 20%.

Some crystal ball-gazers could see the market selloffs coming when Disney revised its growth outlook figures for ESPN. Although the Mouse House execs were happy with record profits, ESPN made modest subscriber losses that became a bellwether and a call to action, sparking a sector sell-off.

This tells you just how important the sports channel is to Disney and to cable operators who use the live sports rights bundler as a major subscriber attraction.

At the same time, Discovery Communications missed its revenue estimates. Add in the advertising slump and there was only one way for the shares to go. However, the most complex reason for the fall is the old chestnut of cord-cutting, which is gathering pace.

In the US, the cable industry is the heart and soul of TV revenues. Viewers have been signing up for large cable bundles for years and years, happy to pay up to $200 a month for many channels that they never even watch (sound familiar?).

For example, satellite pay-TV operator Dish lost 81,000 customers in its second quarter of 2015, compared with 44,000 in the same period last year.

The cause is the various over-the-top (OTT) offers and the smarter – and especially younger – viewers who are saying no to annual subscriptions.

The digital marketplace is serving them with more alternatives: Netflix, obviously, but there other, cheap alternatives such as Sling TV (launched by Dish) and HBO Now (an OTT offer) that are outside of an annual contract and available for much less money.

These ‘skinny’ bundles are more dynamic than the ones on which US media giants have grown so fat. Even Disney may one day have to offer its prized ESPN in a single OTT package (a move it is currently denying).

I see OTT as bigger danger to US media giants than digital – they can (and are starting to) get some of their ad revenue back with digital offers to brands and advertisers. But the US media giants have the legacy business blues: the momentum of their traditional income streams gets in the way of radical change even when change is obviously necessary. It’s a lesson for the UK, for sure. Just ask The Donald.