One writer has suggested that there was a quarrel over money, but there are no grounds to support this.
Another has it that Lady Mary's flirtations or intrigues did not meet with her husband's approval.

Yet another thinks that Montagu found his wife with her sharp tongue, very ill to live with.
The Montagus had been married for seven-and-twenty years; their younger child was now twenty-one.

Since Montagu assisted Lady Mary as a girl with her Latin studies, they do not seem to have had much in common.
Lady Mary cut a figure in the social world; Montagu was a nonentity in political life and seemed content so to be.

Perhaps they were tired of each other, and welcomed a separation that at the outset was intended only to be temporary.

"It was from the customs of the Turks that I first had the thought of a septennial bill for the benefit of married persons," Lady Mary once said to Joseph Spence; and it is more than likely that she would have taken advantage of such an Act of Parliament had it been in existence.
That there was no definite breach is evident from the fact that husband and wife corresponded, though it must be confessed that her letters to her husband are almost uniformly dull, except when the topic is their son.