The number of Cambodians escaping from harsh, violent conditions in their country to Thailand has risen sharply in recent weeks.

What had been a trickle of four or five refugees a week recently has swelled to as many as 20 in one day. More than 300 persons have made the arduous and dangerous trek to Thailand since the increase began.

Opinions vary on what is behind the rise. A Reuter correspondent reporting from a camp for Cambodian refugees in Thailand wrote that the increase was triggered by a renewed purge of supporters of the rightist government defeated by the Communists in April 1975.

Analysts here doubt that that is the cause, saying the purge of officials in the Lon Nol government has never stopped. They believe the increase in refugees has resulted froma lessing in the security along the border. They suspect that military forces have been pulled off patrol duty along the western frontier with Thailand and sent to the east where Cambodian is facing a determined invasion by Vietnam.

They think that about the same number of Cambodians as ever are attempting to flee but in the past many more were caught and killed by border guards than were able to get across the border.

About 15,000 Cambodian refugees are now in four camps is Thailand in addition to more than 100,000 Vietnamese and Laotian refugees in that country.

Informed sources say that there has also been a dramatic rise in the number of Cambodians who have fled into Vietnam presumably to escape the fighting along the border.

In recent months, the observers said, the total has jumped from 100,000 to 300,000. Analysts believe the Vietnamese may be using the refugees to organize a revolutionary movement that will challenge the government in Phnom Penh for control of Cambodia.

Meanwhile, several congressmen are attempting to get the House to join the Senate in urging that the 15,000 Cambodians in Thailand be admitted to the United States under an emergency program. Sources close to the discussions say that the State Department is backing the move.

Officials following the refugee situation say there is mounting pressure for quick action. They note that Cambodia's Deputy Premier Ieng Sary recently visited Bangkok and there are signs that the two countries are finally moving toward normalization of relations. Once this happens, the officials says, the Thais may feel obliged to close the border to refugees and may even forcibly repatriate those already in the country.