He was Maulevrier's Umbra, and must go where his patron went.

It was a hard thing so to think of him, and Mary's heart sank at the thought that Lady Maulevrier's worldly wisdom might have reckoned aright.
'It was very foolish of me to run into the horse,' said Mary, while Mr.
Hammond stood waiting for her to recover herself.
'It was very foolish of Maulevrier to run into you.

If he didn't drive at such a break-neck pace it wouldn't have happened.' Umbra was very plain-spoken, at any rate.
'There's rank ingratitude,' cried Maulevrier, who had turned back, and was looking down at them from his elevated perch.

'After my coming all the way round by Langdale to oblige you with a view of Elterwater.
Molly's all safe and sound.

She wouldn't have minded if I'd run over her.