What do you think prompts her to act strangely when the repairman stops by?
Elisa reacts strangely because she is uncomfortable. In a practical sense, the farm itself is isolated, "closed off..from the sky and from all the rest of the world". The sight of the strange wagon drawn by a burro and a horse must have been startling to say the least.
In a broader, metaphorical sense, Elisa is a woman who, despite her drab, dumpy attire, still has a spirit which is "eager...over-eager, over-powerful". In speaking with the repairman, she finds awakened within her feelings which have long been repressed. Instinctively recognizing that these feelings, representative of dissatisfied longings, are unfamiliar and might be dangerous in the context of the life she has been leading, she at first reacts strangely before tentatively embracing the possiblities. Sadly, Elisa realizes that hope for more fulfillment in her life is false when she sees the chrysanthmum she has given the repairman rudely discarded and crushed at the side of the road.