Reasons vary, but I can give you my own personal experiences (Nitroflare is more hardcore x10):
1) Rapidgator is doing one of their periodic purges. As in, "Hey, its time to give the DMCA complainants a hearty blowjob and act like we give a shit, when we don't since we're making shitloads of cash," purges.
2) You pissed someone off; they outed you; and purge forthcoming.
3) Too much IP address login cycling through VPN's or proxies. VPN works fine with Rapidgator, but if your VPN happens to use any IPs on "The List" (whatever the hell that List is), to quote Robocop (1987), "There will be...trouble." This happens with some forum tech sites--you'll get a screen saying your address is on a list of banned IPs.
4) Way, WAY too many concurrent downloads through IDM, JDownloader, or another internet download manager.
5) Combination of #3 & #4 WILL fuck you most of the time. I say "most" because if you are a yearly subscriber with a long subscription history, you can make-up a good yarn for "plausible deniability," and they will scold you, but turn things back on.
Again, experiences may vary, but that's the lowdown from my end having been a yearly subscriber for a long time.
EPILOGUE: As for Nitroflare....well, management has gotten pissy with VPN, and since I need VPN for my internet to work and to be able to download 1TB+ a month over Sprint, I've had about a gutful of Nitroflare.
IDM's can get your account nuked with Nitroflare. IDMs + VPN either rarely work or don't work with Nitroflare, and its just a matter of time before your subscription is given the death knell.