I mean, it's right there in the article:
The union stated in a press release that this escalation comes in light of the government's failure to disburse 500 shekels 10 days after the last government salary was issued, according to the agreement between the union and the government, in addition to the continued failure to pay salaries in full.
Though it's important to note that this is related to the West Bank PLO's Palestinian Authority, which holds no control of the Gaza strip which is governed by Hamas. The PNA is de facto a US-Israeli delegated government.