John Trapp on Isaiah 28:21

Ver. 21. As in Mount Perazim] See 2 Sam. 5.20. God uſually ſitteth amidſt his people in his mercy-ſeat, or throne of grace: neither ariſeth he to puniſh them till much provoked; and then he may poſſibly deal as ſeverely with them, as he did with the Philiſtines at mount Perazim, or with the Amorites in the valley of Gibeon, Joſh. 10.10. But then he doth his work his ſtrange work, and brings to paſs his act his ſtrange act, i. e. that which is neither his wont nor his delight, Lam. 3.33. Mic. 7.18. Ezek. 33.11. To fall foul upon his people by his plagues and judgements, goeth as much againſt the heart with him, as againſt the hair with them. And beſides by doing this his ſtrange work he maketh way for the doing of his own proper work, 1 Cor. 11.32.