The theme of the episode, as described by Alberto Mielgo, the director for the episode, is about relationships that are based on needs and not love. It's about how those involved keep coming back and back and end up both becoming a victim which is what we see with the cycle in the episode.
Mielgo, while aware of the backlash, says the nudity and sex portrayed in his episode were there for a reason. They are part of a deeper theme, which for him is about the nature of certain relationships and his own religious upbringing as a Catholic.
“This story is very much a representation in the shape of a thriller of difficult types of relationships into which I repeatedly fall … which are not necessarily based on love but on needs, on anything that is like an obsession more than love,” he says.
“In this case, the need and obsession is sex and temptation. These kinds of relationships are usually very difficult to end. For some reason, the lovers, they come back and they come back, and they both become a victim.”
South China Morning Post, Netflix’s Love, Death & Robots in Hong Kong: director of The Witness on his theme and inspiration
I don't think there is an in universe reason for why this is happening though. It seems to be more of a thought-provoking episode than something to be examined in detail and work out why things are happening.