Heading for the rhubarb
Veering dangerously off course, as in, when ploughing the garden with a horse-drawn one- furrow walking plough, which begins to skid to one side, endangering the rhubarb that grows from bulbs perennially in the grass at the edge of the garden.
“Hold ‘er, Newt; she’s heading for the rhubarb!” (farm implements are always feminine … hmmnnn)