The headword, evidently extracted from somewhere, is a feminine noun (see generally LSJ s.v. peri/ptwsis) in the accusative plural. The glossing guarantees this case, rather than nominative plural, and that seems to disqualify Theodoridis' two suggestions (at Photius, Lexicon pi747) for the source of the headword in Basil of Caesarea (PG 29.40a and 31.337c). Instead, a plausible source for the entry as a whole -- with headword and gloss occurring in these forms -- is [RA] Clement of Rome, First Epistle to the Church at Corinth 1.1.1-2: dia\ ta\s ai)fnidi/ous kai\ e)pallh/lous genome/nas h(mi=n sumfora/s kai\ periptw/seis. Clement opens the letter by explaining that tasks on his side which were anticipated by the Corinthians were neglected due to "circumstances and misfortunes occurring both suddenly and in rapid succession"; cf. Clemens Romanus, pp. 1-2.
[1] The gloss is the same form as the lemma; see generally LSJ s.v. sumfora/, and cf. sigma 1407, sigma 1408, and sigma 1409. The headword is identically glossed in the Synagoge, Lexica Segueriana 340.4, and (as noted above) Photius, Lexicon pi747.