In the case of very high buy-in tournaments or even high stakes games, it is relatively common for professional players to play with the financial support of others (who provide part of the necessary funds in exchange for a share of the profits).
Poker as a career is not only about playing well, but also about having great emotional control and knowing how to manage your finances. Another very common thing is the players who become established and start earning serious money, many start spending as if there were no tomorrow, exponentially increasing their consumption and lifestyle, instead of taking advantage to ensure an economic cushion and a huge bankroll. Then come the rough patches and they have to go looking for financial support and stuff.
It is pretty common in the poker world to stake each other for big tournaments. All the top poker players know each other and once you reach your first big payouts it becomes much better to spread your risk than to take it all on your own. In the big tournaments with hundreds of players the chance of winning is small, even for the professional poker player. He might make it into the money, but for reaching the first place you need to be lucky. And it also affects your strategy if you only play to reach the money, or if you want to win the tournament. Sharing some of their profits with other players helps to diversify risks and can lead to payouts even if you got knocked out of the tournament. Daniel Negreanu is doing that even for his fans, you can buy different structured packages before the WSOP each year and make a profit whenever he makes money in a tournament. It kind of similar when people at the final table make a deal among each other to split the money based on their poker rating and then only play for who gets the bracelet. Poker player are business man and are looking for ways to get the most money out of a situation. In tournaments once the rebuy phase is over things can go wrong very fast, imagine getting AA and the chip leader puts you all in preflop only to see his 99 hand hitting a 9 on the turn and you are out of the tournament.