The use of the preterite tense in the main clause triggers the pluperfect subjunctive in the subordinate clause. This is a natural sequence of tenses. The use of the perfect subjunctive (hayan decidido) would be justified with the main clause using the present or the present perfect:
Me extraña que hayan decidido mudarse.
I find it odd that they have decided to move.
Me ha extrañado que hayan decidido mudarse.
I found it/have found it odd that they have decided to move.
With the preterite in the main clause in Spanish you can say:
Me extrañó que decidieran mudarse.
I found it odd that they decided to move.
Me extrañó que hubieran decidido mudarse.
I found it odd that they had decided to move.
I hope this clarified it.
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