500,000 buy-to-let properties to flood market as tax breaks end?

“Half a million buy to let properties could be dumped on the housing market in the next 12 months, potentially driving down house prices as landlords move to avoid a crackdown by George Osborne.”

http://www.dailymail.co.uk/news/article-3429735/Landlords-flood-market-half-MILLION-homes-drive-house-prices-scramble-avoid-George-Osborne-s-buy-let-crackdown.html

Questions: how come there were are so many and what profits were they making up to now? And where will people who can’t afford to buy live?