You, Enemy A, and Enemy B are in a line in adjacent 5-foot squares.
Your Bull Rush score against Enemy A = 25
Enemy A CMD = 23
Enemy B CMD = 23
1) Since 25 - 4 = 21 you don't move both Enemy A and Enemy B back 5-feet, but does Enemy A fall prone because Enemy B is considered an 'obstacle' stopping Enemy A from moving back while standing?
2) If Enemy A would fall prone, could Enemy B move back 5-feet (voluntarily lowering their CMD), to 'catch' them so Enemy A does not go prone?