Here are the 11 reasons why your Upwork account gets suspended:
1: Having fake Identity (fake name or pic).
2: Allowing others to use your account or using others account on same system.
3: Not doing or Failing the ID and phone Verification.
4: Receiving payments Off Upwork.
5: Payment Disputes and Failing to Perform on Your Contracts.
6: Multiple IP/MAC Logins
7: Contacting client outside the Upwork via Skype, emails or personal sites.
8: Forcing or abusing client for the positive feedback.
9: Sending copy paste, non-professional or non-personalized proposals.
10: Sending Many Proposals And Not Getting Hired
11: Poor feedback by clients.
PS: Have more reasons? do add them in comments