How do you read MySQL InnoDB Monitor output?
Please look at this phrase from your question

*** (1) WAITING FOR THIS LOCK TO BE GRANTED: 
RECORD LOCKS space id 0 page no 2712 n bits 264 index `PRIMARY` of table `bt/user_account` trx id 0 984899905 lock mode S locks rec but not gap waiting 
Record lock, heap no 126  


It says index PRIMARY has a shared lock (lock mode S) in Transaction 1

Now, check out this phrase from your question

*** (2) HOLDS THE LOCK(S):     
RECORD LOCKS space id 0 page no 2712 n bits 264 index `PRIMARY` of table `bt/user_account` trx id 0 984900638 lock_mode X locks rec but not gap     
Record lock, heap no 72       
Record lock, heap no 107       
Record lock, heap no 126       


It says index PRIMARY has an exclusive lock (lock mode X) in Transaction 2. Three locks of this type are being held.

I have answered questions like this in the past, even as recent as 4 days ago


Trouble deciphering a deadlock in an innodb status log (Jun 6, 2011)
Will these two queries result in a deadlock if executed in sequence? (Jun 8, 2011)
Reasons for occasionally slow queries? (Jun 14, 2011)
Are InnoDB Deadlocks exclusive to INSERT/UPDATE/DELETE? (Aug 8, 2011)
Concurrently update with limit mysql (Oct 28, 2011)


Here is the bottom line: The PRIMARY KEY, contained in the clustered index (internally called gen_clust_index), can experience intermittent locks when bulk inserting or performing multiple single INSERTs in a transaction. The application should be designed to perform a manual COMMIT after each INSERT. If this is not permissible due to the nature of the application, redesign the app to bulk insert into temp tables before populating the wager table.

As a sidenote, the reason these things do not run in serial fashion is due to the default transaction isolation as well as your app's design when it comes to INSERTs.

Here are the four(4) possible modes:


READ-UNCOMMITTED
READ-COMMITTED
REPEATABLE-READ (default)
SERIALIZABLE


There are probably deeper explanations that the things I have already stated. If there is anyone out there with any further explanation or information I've missed, please chime in.