Because rankings measure whether people can find you, and leads measure whether the finding turned into a call and the call reached you. There are several joints between those two things, and any of them can fail without touching a ranking report.
Start by ruling out the ones that fail silently, because they are the ones nobody is looking at.
The form stopped delivering
A contact form shows a thank-you page whether or not the email behind it was sent. A credential expires, a plugin updates, a mail provider changes a rule, and every submission from that day forward goes nowhere while the site looks perfectly healthy. Submit a test from your phone and watch the inbox. Ten minutes. Here is the full test →
The calls are not reaching the log
Call tracking and phone carriers have settings that can reject calls before they are ever recorded: a spam filter turned up too far, a number flagged by mistake, a forwarding rule pointing at a line nobody answers. From the outside the phone just seems quieter. Call your own tracked number from a phone the system has never seen and confirm it rings through and shows up.
The CRM is counting differently
If your leads number comes from a CRM, check it against the raw sources. Pull the call log and the form submission log for last month and count them by hand. A CRM that is under-reporting against the systems feeding it is more common than you would think, and the dashboard reads normal the whole time.
The listing changed
A ranking report checks where your website ranks. Most local calls come from the map listing, which is a different thing. A suspended listing, a wrong phone number, a category quietly changed, or a competitor's duplicate listing pushing yours down will drop calls with no movement in the website ranking at all. Open your Google Business Profile and read it as a customer would.
The ranking report is measuring the wrong place
A rank checked from your office is the best number you will ever see. If the report tracks one location and your customers live across a whole area, rankings can hold at the office and collapse across town, and the report will not show it. Why that happens →
What is left
If all of that checks out, then it is the market: a competitor's new push, a season, a price change, a review that turned people off. Those are real, and they are worth investigating. But they are the last thing to blame, not the first, because the plumbing fails more often than the market does, and it is the only failure that fixes itself the moment you look at it.