 The voltage across C2 is "V"CC minus "V"BE_Q1 .The moment after Q2 turns on, the right terminal of C2 is now at 0 V which drives the left terminal of C2 to 0 V minus ("V"CC - "V"BE_Q1) or "V"BE_Q1 - "V"CC.From this instant in time, the left terminal of C2 must be charged back up to VBE_Q1.