Question

みみ

下から2行目のonly to returnについてお聞きしたいのですが、ここから後の文構造が分からないので教えて頂きたいです。

Home advantage is a well-documented phenomenon in certain sports,
such as baseball and soccer, where teams are statistically more likely to
win when playing at home than when playing away games. One proposed
explanation for this is the ( 29 ). However, author David Runciman
notes that if this were true, there would have been a much bigger
reduction in home advantage over the past century than there has been,
since luxuries such as first-class flights, fancy hotels, and accompaniment
by therapists for athletes playing away from home have become standard.
Another potential factor is players' familiarity with their home field.
Although sports fields do not differ significantly from one place to
another, Runciman points out that being familiar with even minor aspects
of one's surroundings can "enhance confidence in the performance of
repetitive physical tasks." ( 30 ) when a team moves to a new home
field. The team's home advantage initially goes away, only to return once
the athletes get used to their new surroundings.

Answer 1

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ののののの

その部分しか読んでいないから内容はよくわからないけど
returnはただの自動詞です。
onceは接続詞として使われています。だから後ろに節が続いています。