LSLB uses a "Ping URL" to verify backend periodically, expecting a "200 OK" response. if it failed to connection backend server, or backend server timedout or return something other than "200 OK". LSLB will mark that backend server as "unavailable". LSLB will log the error in error log.
LSLB only give 503 when all backend servers are not reachable.
If LSLB and backend server is over WAN connection, there might be interim network problem.
So, go through error log for the period you getting 503 error, you may be able to find out what was happening. Should set log level to "INFO".