Original title is "Effectiveness and cost-effectiveness of an individualised, progressive walking and education intervention for the prevention of low back pain recurrence in Australia (WalkBack): a randomised controlled trial", which was probably shortened to accomodate the character limit, but the submitter forgot to update the indefinite article.

Someone shortened the original title, removing a word after “an” which had caused the need for “an” rather than “a”.

In other words, the title was shortened incorrectly.

More or less the rule is "an" before vowels or voiceless H.

eg.

An apple.

A banana.

An hour.

A history.

Quite often you see people writing "an historic achievement", but that's incorrect because the H is voiced in historic.

Where it gets a bit vague is stuff like "SQL statement", because some people would say it like "Sequel statement", others will say it like "Ess-Queue-Ell statement".

That's correct. The general rule is to use "an" before a vowel sound, and "a" before a consonant sound. This gets a little fuzzy with acronyms that have several valid pronunciations, but "walking" clearly starts with a consonant sound in this case.

Native English speaker here. "A" is the correct article before the word "walking".