The trailing-slash form is canonical

A 301 from the non-slash URL put you here. That is normal. The test is whether a crawler takes it without looping.

Why the slash is there

Most content management systems, WordPress included, pick the trailing-slash form as canonical, then 301 the non-slash form onto it. So /services and /services/ are one page plus one permanent redirect, not two pages. The behavior is correct, and every crawler has to cope with it.

How crawlers trip

Now suppose a crawler strips trailing slashes to deduplicate URLs. It rewrites the canonical /services/ as /services, fetches that, and gets a 301 back to /services/. Strip the slash again, queue /services again, repeat. A loop detector finally gives up with a 508. Two habits avoid the trap. Leave trailing slashes alone when normalizing URLs, or, when the 301 leads to the slash form, record that URL as already seen. Either way the page is fetched once and never queued again.