A short, somewhat portly man playing baseball is stepping onto a base as the player from the other team at that base is appearing to catch the ball.
A short man stole a base before any of the opposing team's players could react
The first sentence only says the man is stepping onto base.  We can't argue he stole a base because there are several scenarios that can explain why he is stepping onto base.
exists e1 e2 p1 s1 s2 s3 x1 x2 x3 x4 x5 x6 x7.(n_ball(x7) & -(x6 = x6) & n_team(x6) & n_player(x5) & exists e3 p2.(Topic(e3,p2) & Actor(e3,x5) & v_appear(e3) & exists e4.(Theme(e4,x7) & Actor(e4,x5) & v_catch(e4)) & from(x5,x6) & at(x6,x4)) & n_base(x4) & n_base(x4) & as(e2,p1) & onto(e2,x4) & Actor(e2,x1) & v_step(e2) & Theme(e1,x3) & Actor(e1,x2) & v_play(e1) & n_baseball(x3) & (x1 = x2) & n_man(x1) & a_portly(s3) & Theme(s3,x1) & r_somewhat(x1) & a_,(s2) & Theme(s2,x1) & a_short(s1) & Theme(s1,x1))
exists e1 p1 s1 s2 x1 x2 x4 x5.(n_player(x5) & of(x5,x4) & n_team(x4) & a_opposing(s2) & Theme(s2,x4) & all x3.(n_thing(x3) -> exists e2 p2.(exists e3.(Actor(e3,x3) & v_react(e3)) & Topic(e2,p2) & Actor(e2,x3) & v_could(e2) & of(x3,x5))) & before(e1,p1) & Theme(e1,x2) & Actor(e1,x1) & v_steal(e1) & n_base(x2) & n_man(x1) & a_short(s1) & Theme(s1,x1))