1) The ads are now displayed so prominently that everybody has to notice them. Earlier, they where less prominent and more clearly marked as ads. So there is probably a shift in demographics. Less dedicated buyers and more people who just stumble upon the ad.

2) On tablets and phones people might a) misclick more and b) surf more without the intend to buy.

then cpc become kink when everyone realized advertising based in demographic was not generating returns for the brands, because in the internet nobody knows you are a dog.

so everyone spend all their money on cpc, because they were told that was the metric to follow. until they realized it also does not result in conversion.

then they start spending all their money on conversion, and realized they had to give away all their profit per conversion.

then they briefly guessed that going back to before cpc was good, and everyone dumped their money on ad networks and paid cents for million of impressions god knows where. turns out those impressions were all in russian warez sites and porn.

now everyone is spending all their money on viewable impression. until they realize no browser allows for a real world implementation of this, and even if they did, in the end it is exactly the same as buying the more expensive placements on impression based publishes. but this is still beginning, so we are here. everyone is dumping their money on this this week

oh and every step, after the premium bands got tired of the new thing, the publishers being stuck with it, start a race to the bottom on price.