I'm trying to use the following F# code to access a Xaml control.

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    let (?) (source:obj) (s:string) =
        match source with 
        | :? ResourceDictionary as r ->  r.[s] :?> 'T
        | :? Control as source -> 
            match source.FindName(s) with 
            | null -> invalidOp (sprintf "dynamic lookup of Xaml component %s failed" s)
            | :? 'T as x -> x
            | _ -> invalidOp (sprintf "dynamic lookup of Xaml component %s failed because the component found was of type %A instead of type %A"  s (s.GetType()) typeof<'T>)
        | _ -> invalidOp (sprintf "dynamic lookup of Xaml component %s failed because the source object was of type %A. It must be a control or a resource dictionary" s (source.GetType()))

This is from Daniel Mohl's excellent F# for Windows Phone template. 

I've created a class to basically read the accelerometer and trigger an event when the phone is shaken.  The event gets raised as expected but for some reason it's spawned in a second thread--which leads the CLR to throw an "invalid cross-thread access" exception when the event handler attempts to execute this code.  The exception is thrown on the source.FindName(s) call. I can see a second thread of execution--which kind of surprises me because I didn't specifically spawn a secondary thread.  I mean I didn't explicitly call async or do anything else that I can think of that would cause a secondary thread of execution to start. 

So it seems like there are a few approaches I could take:

1. I could try to figure out why I'm spawning a secondary thread when I haven't specifically requested it and modify the code to prevent a secondary thread of execution from being spawned.
2. I could try to modify the (?) function to account for multiple threads of execution but I'd really like to understand why I'm getting a second thread started.

I think probably the second approach is best but I'd really like to understand what I'm doing that's causing a secondary thread to spawn. I realize how hard that is to answer without specific code but I don't mind researching this if someone can point me in the right direction. I believe this has something to do with the Windows 7 Phone platform because the code is, as far as I can tell, pretty much the idiomatic way to bind a Xaml control with F# code.

Any thoughts, comments, suggestions would be greatly appreciated.