In Black Beauty, what did The Earl do that tempted Reuben to break his promise?
Most of The Earl's interaction with Reuben was through York. Reuben possessed a temperament that moved him closer to drink. This is something that is noted in the text in how Reuben had a "love of drink." York understood this. He did his best to conceal Reuben's penchant for drinking from the Earl. This worked until one evening, "when Reuben had to drive a party home from a ball he was so drunk that he could not hold the reins, and a gentleman of the party had to mount the box and drive the ladies home." The event caught the attention of the Earl who fired Reuben immediately. Through York's persuasion, Reuben was rehired. For his part, Reuben was indebted to the Earl:
...the man had promised faithfully that he would never taste another drop as long as he lived there. He had kept his promise so well that York thought he might be safely trusted to fill his place while he was away, and he was so clever and honest that no one else seemed so well fitted for it.
He was able to control his habit of drinking, in large part because of the fear of angering the Earl again. The presence of the Earl and the persistent belief of York in Reuben enables his promise to not drink again.
However, when the Earl left and took York with him, Reuben was tempted to break his promise not to drink. There was no one watching over him. The lack of an external check against his propensity for drink is what tempted him to go into town and drink alcohol to the point of inebriation. Black Beauty notices this as Reuben's voice was "loud" and "rough." At this point, it has become clear that Reuben succumbed to the temptation of drink and broke his promise. By leaving and taking York with him, the Earl created the temptation for Reuben to go back on his word and drink. It is this condition that enables the condition for temptation, something that Earl embraces in breaking his promise to not drink alcohol.