Because their conflict has been constructed indirectly, their overwrought reconciliation seems hysterical (hysteria being a disorder characterized by violent emotional outbreaks, irrationality, and frenzy; historically hysteria has been associated with women, being initially diagnosed as a uterine illness). This hysterical quality also appears in Sonya's inexplicable jubilation, and Yelena's manic impulse to play the piano. Their encounter seems to involve an explosion of affect (being one of the few instances of joy in the play, albeit one that is preemptively stifled by the professor off-stage) separated from any clear cause or idea. This split between affect and idea is disconcerting to say the least.
Finally, since Chekhov's character notes are relatively sparse, we might linger on Astrov's characterization of Yelena: as an idle woman who charms with her beauty and offers little else. Yelena seems to understand herself in similar terms: as she tells Sonya, she has been but an "incidental character" in her love affairs, her home, her studies, and onward, her remark perhaps recalling Voynitsky's repeated accusations that she is not really living. Unlike other characters, alienated by age (Astrov) or displacement (Serebryakov), Yelena's sense of alienation lies in her being inconsequential in her own existence.
[Additional note: Yelena and Sonya's reconciliation refers to a custom of the Russian provinces. Literally, they say "Let us drink to Brudershaft (fellowship) Well, that means—na ty?" (or, "we will use the familiar second person when we address each other?"). This drink to fellowship requires the linking of arms and touching of glasses.]