The circumstances surrounding Edgar H. Griffiths' surprise announcement that he would resign July 1 as chairman of RCA Corp. remain something of a mystery, although top officials of the communications conglomerate insist that the strong-willed, 59-year-old chief executive was not forced out.

But some highly placed industry sources in a position to know insist that Griffiths was told by the board of directors last week that the company needed a new chairman -- one who could relate better to Washington regulators and Wall Street analysts and who was not tainted by the embarrassing public firings last summer of RCA President Maurice Valente, Griffiths' hand-picked successor, and National Broadcasting Co. Chairman Jane Cahill Pfeiffer.

Except for the two firings and the inability of NBC to crawl out of third place in the prime-time television network ratings, despite Griffiths' hiring of programming whiz Fred Silverman to head the network, Griffiths had performed more than respectably as chief of RCA.

The conglomerate has achieved higher earnings in nearly all its operations except broadcasting, and Griffiths has sold off operations that did not seem to fit in the RCA plan and acquired others -- such as C.I.T. Financial Corp. -- that did.

"He's been accused of having short-term profit goals. But he's been a superb chief executive, and when you look back on the divestitures, the acquisition of C.I.T. and the overall performance, you see what he has done," said Frank Olson, head of RCA's Hertz Corp. subsidiary. "He had the courage to launch an entry into the video disc market and each year made bigger and bigger investments in research and development, which he wouldn't have to do if he'd been motivated merely by short-term goals. It's not a loyalty comment; it's a fact."

Griffiths will be replaced by Thornton Bradshaw, president of Atlantic-Richfield Oil Co., who formally informed Arco's board of directors of the job change at a meeting in Los Angeles this morning. RCA had planned to make the announcement to the public this afternoon following a lunch Griffiths had with the 14 top operating officers of the company.

But the plans went awry when Newsweek learned of the impending shift and issued a press release Saturday afternoon announcing that it would publish this week that the board had demanded Griffiths' resignation.

RCA rushed out a statement Saturday night saying that Griffiths, who turns 60 on June 23, had asked to be relieved of his duties on July 1. Griffiths will remain a member of the RCA board in the newly created position of chairman of the Finance Committee.

RCA officials said that Griffiths long had evinced a desire to retire in 1981 and that -- after a special board committee created after the Valente firing and charged in part with finding a successor to Griffiths found in early January that Bradshaw would take the job -- Griffiths and the board reached an agreement that resulted in Griffiths' July 1 resignation. Sources said Griffiths and the board agreed last November that the chief executive would continue to collect his $450,000-a-year salary for five years as a consultant to RCA.

Bradshaw, a polished oil executive who likes the social and political scene that Griffiths shunned, is 63 and has been a member of RCA's board for nine years.

Griffiths is known as something of an autocrat and has had stormy battles with at least some outside directors of the board -- including Donald Smiley, retired chief executive of R. H. Macy Co., who headed the special RCA board committee.

But some board insiders say that although many directors had their differences with Griffiths -- and are upset about the Valente hiring and firing, the Pfeiffer firing and the ratings and profits problems at NBC -- they tried to convince Griffiths to stay on until the end of 1981 at least. -