The default lead owner is where Salesforce sends a lead when nothing else decided. A few leads there is normal. A pile is a symptom, and the cause is one of a short list.
1. No entry matched
The lead did not satisfy any entry. Look at the leads that landed there and compare their country, state, source and size fields with the entries. See assignment rule not working.
2. The rule did not run
Imports, integrations and the API only run the rule when asked. See leads from an import or an integration.
3. The form sends different values
A form that posts a country code to a rule written for names matches nothing. See routing by country.
4. The default owner is a person who left
If the default owner was deactivated, leads still go there and nobody works them.
5. A flow reassigns them
A flow that sets the owner to a fixed user sends everything to one person, whatever the rule decided. See a flow overriding the rule.
Check it yourself
SELECT LeadSource, COUNT(Id) FROM Lead WHERE IsConverted = false AND OwnerId = '<default owner id>' GROUP BY LeadSource ORDER BY COUNT(Id) DESCGroup by the field you suspect, such as source, country or how the lead was created. A cluster on one value shows which path is skipping the rule.
Have it checked every night instead.
Routing Forensics connects as a read-only user and checks this, and the rest of your routing, against your real leads. Register interest and you will be first to get a free health report: what is leaking, where, and what it is worth.
The free scan cannot see this one: it reads assignment rules, flows and queues only. Connected, it is checked every night.