In rough order of how often we see them when someone forwards us a hosting invoice.
Most common
The introductory term ended
How to check: The increase lands exactly at a 12, 24, or 36 month boundary from when you signed up, and the plan name has not changed. The advertised monthly figure that got you in the door was for the first term.
The fix: Nothing about your site caused this and nothing about your site will fix it. Either accept the standard rate, negotiate at renewal, or move to a host that charges the same at renewal as it does on day one.
Very common
Add-ons stacked up
How to check: Count the non-hosting lines: domain privacy, site backup, a security or malware scanner, an SEO tool, extra email boxes, a dedicated IP, a paid SSL. Each is small. Together they are frequently larger than the hosting itself.
The fix: Cancel anything you cannot describe the benefit of. Replace paid SSL with the free automatic certificate. Keep backup and malware protection in some form, but be clear whether you are buying it twice, once from the host and once from a plugin.
Common
Domain renewals bundled in
How to check: A domain renewal on the same invoice, often with privacy protection alongside it. Registration is usually cheap for the first year and normal price after, and privacy is sometimes free and sometimes not.
The fix: This is a legitimate cost and you should pay it. Just know it is a domain charge, not a hosting increase, and it is annual rather than monthly. Do not cancel it to save money.
Less common
The plan was changed for you
How to check: The plan name on the new invoice is different from the old one, or there is a note about an upgrade you do not remember requesting. Sometimes this follows a resource-limit warning email.
The fix: Ask what triggered it and whether the old plan is still available. If the site genuinely outgrew the plan, that is a real conversation about resources. If it did not, ask to be put back.
Less common
You actually outgrew the plan
How to check: Overage charges, bandwidth or storage warnings, or the site slowing down under traffic. Real growth looks like this, and it is the one case where paying more is the right answer.
The fix: Move to a plan sized for the traffic. Before you do, check the storage is real content rather than years of accumulated backups sitting in the hosting account.