Investors in IndyMac’s mortgage securities backed by Alternative A loans were surprised by an unexpected surge in prepayment speeds in August, Wall Street analysts report. Prepayment speeds on certain IndyMac Alt A securities jumped by as much as 25 to 50 percent conditional prepayment rate, or CPR, according to a recent research note from Bear Stearns MBS analyst Bruce Kramer. Comparable Alt A deals from Countrywide and GMAC Residential Funding Corp. were up only 2-3…