The simple and underlying reason is that the conflict between the Republicans and Democrats is getting out of control. Their biggest clash is on the immigrant issue. This is the second time USA government shut down under Donald Trump, which equals him with Bill Clinton who, too, had to take the brunt of shutdown twice.
However, if you’re looking for a more technical answer, here it is—basically, USA government gets funded through laws the Congress must pass con-sensually. But this time, Congress failed to pass the government funding law, citing that they should first wait for the earlier ones to expire. Senator Paul Rand took a strict stance, contending that the deal would lead to a huge fiscal deficit in the federal budget by increasing the defense and non-defense budget by $300 billion.
At the end of the day, the funding law did get passed narrowly by a margin of 240 to 186. This was the 19th time in modern history that USA government was shut down.