تَأَخَّرَ الْعَمَلُ. اشْتَدَّ الْمَطَرُ.
The work was delayed. The rain grew intense.
The first statement naturally raises the question, 'Why was the work delayed?' If the second statement supplies the answer, it functions as explanatory resumption (istiʾnāf bayānī), and no wāw is required between the expressed clauses.
In another setting the speaker might intend two independent, coordinate reports. The fact that both concern the same project does not settle the issue. What matters is whether the second statement answers an implied question or adds an independent proposition.